[ENH] Property testing for metadata indices (#1794)

## Description of changes

*Summarize the changes made by this PR.*
 - New functionality
- Make MetadataIndex and BlockfileMetadataIndex properly generic. Let
the type system do the work!
- Property testing for metadata indices. I learned a lot about the
borrow checker :0)

## Test plan
*How are these changes tested?*

- [x] Tests pass locally with `pytest` for python, `yarn test` for js

## Documentation Changes
*Are all docstrings for user-facing APIs updated if required? Do we need
to make documentation changes in the [docs
repository](https://github.com/chroma-core/docs)?*