Webdataset dataset builder
_The documentation is not available anymore as the PR was closed or merged._ I added an error message if the first examples don't appear to be in webdataset format
```
"The TAR archives of the dataset should be in Webdataset format, "
"but the files in the archive don't share the same prefix or the same types."
``` @mariosasko could you review this ? I think it's fine to have webdataset as an optional dependency for now, then depending on usage and user feedbacks see if it makes sense to have our own implementation or not I just removed the dependency on `webdataset` @mariosasko :) took your comments into account, lmk if you see anything else
Allow `load_dataset` to support the Webdataset format.

It allows users to download/stream data from local files or from the Hugging Face Hub.

Moreover it will enable the Dataset Viewer for Webdataset datasets on HF.

## Implementation details

- I added a new Webdataset builder
- dataset with TAR files are now read using the Webdataset builder
- Basic decoding from `webdataset` is used by default, except unsafe ones like pickle
- HF authentication support is done with `xopen`

## TODOS

- [x] tests
- [x] docs