Support complex feature types as `features` in packaged loaders 
{'login': 'mariosasko', 'id': 47462742, 'node_id': 'MDQ6VXNlcjQ3NDYyNzQy', 'avatar_url': 'https://avatars.githubusercontent.com/u/47462742?v=4', 'gravatar_id': '', 'url': 'https://api.github.com/users/mariosasko', 'html_url': 'https://github.com/mariosasko', 'followers_url': 'https://api.github.com/users/mariosasko/followers', 'following_url': 'https://api.github.com/users/mariosasko/following{/other_user}', 'gists_url': 'https://api.github.com/users/mariosasko/gists{/gist_id}', 'starred_url': 'https://api.github.com/users/mariosasko/starred{/owner}{/repo}', 'subscriptions_url': 'https://api.github.com/users/mariosasko/subscriptions', 'organizations_url': 'https://api.github.com/users/mariosasko/orgs', 'repos_url': 'https://api.github.com/users/mariosasko/repos', 'events_url': 'https://api.github.com/users/mariosasko/events{/privacy}', 'received_events_url': 'https://api.github.com/users/mariosasko/received_events', 'type': 'User', 'site_admin': False}
[]
[]
_The documentation is not available anymore as the PR was closed or merged._
This PR adds `table_cast` to the packaged loaders to fix casting to the `Image`/`Audio`, `ArrayND` and `ClassLabel` types. If these types are not present in the `builder.config.features` dictionary, the built-in `pa.Table.cast` is used for better performance. Additionally, this PR adds `cast_storage` to `ClassLabel` to support the string to int conversion in `table_cast` and ensure that integer labels are in a valid range.

Fix https://github.com/huggingface/datasets/issues/4210

This PR is also a solution for these (popular) discussions: https://discuss.huggingface.co/t/converting-string-label-to-int/2816 and https://discuss.huggingface.co/t/class-labels-for-custom-datasets/15130/2

TODO:
* [x] tests
{'url': 'https://api.github.com/repos/huggingface/datasets/issues/4364/reactions', 'total_count': 1, '+1': 0, '-1': 0, 'laugh': 0, 'hooray': 1, 'confused': 0, 'heart': 0, 'rocket': 0, 'eyes': 0}
{'url': 'https://api.github.com/repos/huggingface/datasets/pulls/4364', 'html_url': 'https://github.com/huggingface/datasets/pull/4364', 'diff_url': 'https://github.com/huggingface/datasets/pull/4364.diff', 'patch_url': 'https://github.com/huggingface/datasets/pull/4364.patch', 'merged_at': '2022-05-31T12:16:31Z'}