Allow loading/saving of FAISS index using fsspec
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.
