Webdataset dataset builder
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