[ENH] Make block and sparse index caches use persistent type.

[rescrv]

End-to-end tests confirm that with this change foyer will initialize the disk path (assuming it's writable) and instantiate a disk. A memory cache will also instantiate.

Note that the PersistentCache type means that the key and value implement StorageKey and StorageValue, so it's totally acceptable for every non-persistent cache to implement the trait. This means that any place that takes a persistent cache will also take the non-persistent versions. It's trait logic, not "is a".

Description of changes

Summarize the changes made by this PR.

• Improvements & Bug fixes
  • ...
• New functionality
  • ...

Test plan

How are these changes tested?

• Tests pass locally with pytest for python, yarn test for js, cargo test for rust

Documentation Changes

Are all docstrings for user-facing APIs updated if required? Do we need to make documentation changes in the docs repository?

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