Issue title: jxl Content-Encoding
 Issue body: Saluton TAG!

I'm requesting a TAG review of "jxl Content-Encoding".

One of the features of JPEG XL is byte-wise lossless JPEG image repacking. On average, an encoded image is 22% smaller. We propose it as a new HTTP "Content-Encoding".

  - Explainer: https://github.com/google/brunsli/blob/master/explainer.md
  - Specification URL: https://arxiv.org/pdf/1908.03565.pdf
  - Tests: N/A, fetch API forbids altering "Accept-Encoding" header
  - Security and Privacy self-review: https://github.com/google/brunsli/blob/master/security-privacy-questionnaire.md
  - GitHub repo: https://github.com/google/brunsli
  - Primary contacts:
      - Eugene Kliuchnikov (eustas), Google
  - Organization(s)/project(s) driving the specification: Google, JPEG XL
  - Key pieces of existing multi-stakeholder review or discussion of this specification: -
  - External status/issue trackers for this specification: https://www.chromestatus.com/feature/5678152091172864

Further details:

  - [x] I have reviewed the TAG's [API Design Principles](https://w3ctag.github.io/design-principles/)
  - Relevant time constraints or deadlines: 2020Q3
  - The group where the work on this specification is currently being done: ?
  - The group where standardization of this work is intended to be done: ?
  - Major unresolved issues with or opposition to this specification: none
  - This work is being funded by: ?

We'd prefer the TAG provide feedback as:
  🐛 open issues in our GitHub repo for **each point of feedback**

 Comments: 
 Comment 0: Is this not specific to JPEG despite the name? If it is specific to JPEG it seems a bit weird to use `Content-Encoding` for this.
 Comment 1: The format could be used for encoding any type of data. So it fits the definition of HTTP Content-Encoding.
 It is unlikely that the compression ratio will surpass the compression ratio of gzip / brotli unless content is a JPEG file.

The volume of JPEGs traffic is bigger than total volume of HTML, CSS and JS traffic.
jxl Content-Encoding will cover the weaknesses of gzip and brotli for such kind of traffic.
 Comment 2: In the specification, I see this:
`If the codestream starts with bytes {0xFF, 0xD8, 0xFF, 0xE0}, the decoder shall decode a JPEG1 as specified in ISO/IEC 10918-1:1993(E). Otherwise, if the codestream starts with bytes {0x0A, 0x04, 0x42, 0xD2, 0xD5, 0x4E}, the decoder shall reconstruct the original JPEG1 codestream as specified in Annex M. Otherwise, the codestream shall be structured as shown in Table 1; the syntax is described in Annex A.`

So it seems linked heavily to JPEG processing, at least for the `{0x0A, 0x04, 0x42, 0xD2, 0xD5, 0x4E}` case. 

Is there a way to recognise the generic encoding, if this applies to other formats, or just the signature above is used and only for JPEG? 
If it is only for JPEG, then a new media type would be easier to deploy than going through the [IETF Review](https://tools.ietf.org/html/rfc5226#section-4.1) which is needed to update https://www.iana.org/assignments/http-parameters/http-parameters.xhtml , although the Content coding option is not wrong.

@annevk mime sniff applies only after Content-Encoding is processed, so it shouldn't be an issue if the content coding option is used (ie: no need to add a new signature for a repacking of jpeg)
 Comment 3: It is proposed only to process `{0x0A, 0x04, 0x42, 0xD2, 0xD5, 0x4E}` case as Content-Encoding.
So, you are correct: only JPEG1 streams could be expressed in compressed form; arbitrary data could be "framed" (see `M.5.1`) as well.

`image/jxl` is already registered as provisional standard media type (see https://www.iana.org/assignments/provisional-standard-media-types/provisional-standard-media-types.txt). This media type corresponds to fully-fledged JPEG XL encoding.

Perhaps explainer lacks the motivation, why we want to add the lossless past of JPEG XL as Content-Encoding.
Going to update  explainer with the following information soon:

```
Pros
 + faster JPEG rendering of cached content: JPEG uses less CPU compared to modern image formats
 + compatibility: there are a lot of expensive cameras / smartphones that produce JPEGs; it will take at least 10 years to transition to the new format
 + seamless experience: when user saves the image (JPEG) it is guaranteed to be supported by existing software
 + lower deployment risks: Content-Encoding story is similar to Brotli, so it would not cause new risks
 + risk-free serving: no re-review is required for transcoded images, as those are guaranteed to be exactly the same
 + easier for webmasters: no image optimization stack is involved; nginx / Apache plugins will support the new encoding with the zero effort
 + exactly same pixels for legacy clients and new clients (both are decoding the same jpeg)
 + same behaviour: same progression

Contras
 - systems for image optimization / re-coding already exist (cloudinary, cloudflare, akamai, etc.): adding a new image format looks easier for those than adding new Content-Encoding
 - adding "save-as" in browsers would solve the "seamless experience" point (however it would unexpectedly for users loose transparency / animation / etc. features)
```


 Comment 4: Note that `Content-Encoding` is a property of the payload, so saving-as will save the `jxl` version, as opposed to `Transfer-Encoding` which has the drawback of being hop-by-hop. So you will probably need a specific 'save-as after resolving content-encoding' while brunsli is available everywhere.

Also conneg done via plugins like that are often either not really doing a proper job, as they don't have access to all the axis available for content negotiation, so listing those plugins as "pros" is debatable.

The real "pro" is that the jpeg/jxl/jpeg conversion is lossless, and that is only what matters when asking for a new content coding, even if it is not generic but tight to a specific media type (there are some examples in the [IANA listing of content codings](https://www.iana.org/assignments/http-parameters/http-parameters.xhtml))
 Comment 5: Looked at this in our VF2F with @rossen and @LeaVerou, we're concerned that while this is perfectly valid, it seems to be adding a lot of complexity relative to the value. The same end goal could be achieved by the server having both jpeg and jxl versions of the file on the server and using content negotiation, rather than encoding negotiation. Doing so prevents the server from having to constantly re-encode the source file and spending CPU cycles and energy.
 Comment 6: > Note that `Content-Encoding` is a property of the payload, so saving-as will save the `jxl` version

'Content-Encoding' is a property of _network_ payload. Most browsers decode the network payload first and then provide it to users. Example: load a text file with `Content-Encoding: br` (or `gzip`). "Save" will save plain text...
With jxl Content-Encoding the output is a normal JPEG file, so users don't have to have support of new format to use the saved images.
 Comment 7: @plinss The benefit of jxl Content-Encoding is that users will be able to "save" images right away and those will be normal JPEGs, not jxl. Also with "Content-Encoding" we make a promise that users will be receiving the _same_ content without any possible conversion loss.
 Comment 8: Thanks for the responses. @ylafon and I took a look a this in today's breakout session and we don't see any issues with this proceeding. We think it's to be seen how much this gets taken up in the wild vs just switching to jxl content types. At some point this may be a target for deprecation if usage numbers are low, but given the amount of jpeg usage this may have some demonstrable short-term benefits.
{'source': 'GithubIssues', 'repo_name_with_owner': 'w3ctag/design-reviews', 'issue_id': 669138022, 'repo_stars': 235.0}