Add Video feature, videofolder, and video-classification task
The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/datasets/pr_5339). All of your documentation changes will be reflected on that endpoint. @lhoestq I think I need some serious help with the tests 😅...I started this locally but it got too time consuming.

One issue I remember running into is with lossless audio encoding/decoding. I started thinking of using the underlying Audio feature instead of PyAV so I didn't have to rewrite similar logic here...but assumed that would turn into a mess w/ underlying logic Are you still planning to work on this? I'm closing this PR. Feel free to reopen it if necessary.
This PR does the following:

- Adds `Video` feature (Resolves #5225 )
- Adds `video-classification` task
- Adds `videofolder` packaged module for easy loading of local video classification datasets

TODO:
  - [ ] add tests
  - [ ] add docs