Issue title: Delegated sign out issues
 Issue body: I am having issues with delegated sign out.  I'd like to redirect the signout request to Auth0. This flow works fine on the legacy portal. 

Any guidance on how to handle the preflighted CORS request?  

 Comments: 
 Comment 0: Hi Adam, can you please tell in more detail at what point you're facing the CORS issue? How the flow looks like?
 Comment 1: Here is an overview of the requests I see in dev console upon clicking "sign out":

```
APIM      |      my server     |     auth0
    ----[OPTION]---->
    <---[200]--------
    ----[GET]------->
    <---[302]--------
    -----------[OPTION]-------------->
    <----------[200]------------------
[XHR FAILURE]
```

 Comment 2: Thank you for the details. We will fix it.
 Comment 3: Any progress on this? 

Alternatively, if you can provide some documentation around the expected signout flow, maybe I can fix things on my end. 
 Comment 4: @adamstruck We'll soon implement a fix, the ETA is 2-4 weeks.
 Comment 5: I wanted to add that this is also an issue when delegating sign out requests to Azure AD B2C.  The `https://<tenant-name>.b2clogin.com/<tenant-name>.onmicrosoft.com/<policy-name>/oauth2/v2.0/logout` endpoint responds the OPTIONS request with 404 
 Comment 6: The fix is included in the release we're rolling out now. ETA for full deployment is ~2 weeks.
{'source': 'GithubIssues', 'repo_name_with_owner': 'Azure/api-management-developer-portal', 'issue_id': 632077050, 'repo_stars': 372.0}