Add support for metadata files to `imagefolder`
This PR adds support for metadata files to `imagefolder` to add an ability to specify image fields other than `image` and `label`, which are inferred from the directory structure in the loaded dataset.

To be parsed as an image metadata file, a file should be named `"info.csv"` and should have the following structure:
```
image_id,some_col1_name,some_col2_name
rel/path/to/image1.jpg,image1_col1_value,image1_col2_value
rel/path/to/image2.jpg,image2_col1_value,image2_col2_value 
...
```

This is how the resolution works:
```
- path/to/imagefolder/directory
  - info.csv
  - 10.jpg # referenced as 10.jpg in "info.csv"
  - Cat
    - 0.jpg  # referenced as Cat/0.jpg in "info.csv"
    - 1.jpg  # referenced as Cat/1.jpg in "info.csv"
  - Dog
    - 0.jpg  # referenced as Dog/0.jpg in "info.csv"
    - 1.jpg  # referenced as Dog/1.jpg in "info.csv"
```

Open questions:
1. IMO it makes more sense to store image metadata as JSON Lines than CSV. CSV is sufficient for textual metadata but not the best for representing bounding boxes, for instance. Also, JSON Lines is more strict, which is good in this case (CSV supports various delimiters, the header line is optional, etc., so it's easier to enforce rules on JSON Lines that it's on CSV)
2. A better name for the `image_id` column, which contains image identifiers? Maybe `image_file` or `image_filename`?
3. WDYT about making `with_metadata=True` the default behavior if the loaded repo/directory contains an `info.csv` file?

An example repository: https://huggingface.co/datasets/mariosasko/PetImages. Can be loaded by installing `datasets` from the PR branch and running `load_dataset("mariosasko/PetImages", with_metadata=True)`.

cc: @abhishekkrthakur (this PR should address https://huggingface.slack.com/archives/C02JB9L6JKF/p1645450017434029?thread_ts=1645157416.389499&cid=C02JB9L6JKF)

TODOs:
- [x] Test 
- [x] Metadata file nesting
   ```
    - path/to/imagefolder/directory
      - info.csv
      - 10.jpg
      - Cat
        - info.csv  # should have higher precedence in this directory than the top-level info.csv, but we choose the first "eligible" metadata file currently
        - 0.jpg
        - 1.jpg
   ```