Allow loading/saving of FAISS index using fsspec
The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/datasets/pr_5526). All of your documentation changes will be reflected on that endpoint. Thanks for the quick review! I updated the code with your suggestion Thanks for the quick review @albertvillanova! I updated the code with your suggestions
Fixes #5428

Allow loading/saving of FAISS index using fsspec:
1. Simply use BufferedIOWriter/Reader to Read/Write indices on fsspec stream.
2. Needed `mockfs` in the test, so I took it out of the `TestCase`. Let me know if that makes sense.


I can work on the documentation once the code changes are approved.
