Add support for `Audio` and `Image` feature in `push_to_hub`
> Cool thanks !
> 
> Also cc @patrickvonplaten @anton-l it means that when calling push_to_hub, the audio bytes are embedded in the parquet files (we don't upload the audio files themselves)

Just to verify quickly the size of the dataset doesn't change in this case no? E.g. if a dataset has say 20GB in size when stored in `.mp3` format it could have up to 100GB when stored in WAV. But since we are just taking the bytes here a 20GB .mp3 dataset would also have 20GB when stored in parquet no? @lhoestq  I've addressed your comments. Additionally, I've modified `cast_storage` to account for possible null (`None`) values.

@patrickvonplaten Yes, the dataset size stays the same (at least because Parquet files are compressed). Feel free to merge if it's all good to you :)
Add support for the `Audio` and the `Image` feature in `push_to_hub`. 

The idea is to remove local path information and store file content under "bytes" in the Arrow table before the push.

My initial approach (https://github.com/huggingface/datasets/commit/34c652afeff9686b6b8bf4e703c84d2205d670aa) was to use a map transform similar to [`decode_nested_example`](https://github.com/huggingface/datasets/blob/5e0f6068741464f833ff1802e24ecc2064aaea9f/src/datasets/features/features.py#L1023-L1056) while having decoding turned off, but I wasn't satisfied with the code quality, so I ended up using the `temporary_assignment` decorator to override `cast_storage`, which allows me to directly modify the underlying storage (the final op is similar to `Dataset.cast`) and results in a much simpler code. 

Additionally, I added the `allow_cast` flag that can disable this behavior in the situations where it's not needed (e.g. the dataset is already in the correct format for the Hub, etc.)

EDIT:
`allow_cast` renamed to `embed_external_files`