Stream private zipped images
_The documentation is not available anymore as the PR was closed or merged._ oops looks like some tests are failing sorry, will fix them tomorrow

EDIT: not today but asap hopefully cc @mariosasko this is ready for review, let me know what you think !
As mentioned in https://github.com/huggingface/datasets/issues/4139 it's currently not possible to stream private/gated zipped images from the Hub.

This is because `Image.decode_example` does not handle authentication. Indeed decoding requires to access and download the file from the private repository.

In this PR I added authentication to `Image.decode_example` via a `token_per_repo_id` optional argument. I first wanted to just pass `use_auth_token` but a single `Image` instance can be responsible of decoding images from a combination of several datasets together (from `interleave_datasets` for example). Therefore I just used a dictionary `repo_id` -> `token` instead.

I'm getting the `repo_id` from the dataset builder (I replaced the `namepace` attribute with `repo_id`)

I did the same for `Audio.decode_example`

cc @SBrandeis @severo 