[FEATURE] Allow users to configure specific types of vector index for Postgres

[georgeh0]

The request comes from #1043 originally.

There're different vector index types, e.g. Postgres support IVFFlat / HNSW. Currently we default to HNSW. We can expose an option to change the default and use IVFFlat instead of HNSW. We should also expose extra options for each type, e.g. m and ef_construction for HNSW.

---

❤️ Contributors, please refer to 📙Contributing Guide.
Unless the PR can be sent immediately (e.g. just a few lines of code), we recommend you to leave a comment on the issue like I'm working on it or Can I work on this issue? to avoid duplicating work. Our Discord server is always open and friendly.

[banrovegrie]

Hi @georgeh0, I was trying to look into this and the other issue at #1043. Put up a draft PR at #1050. Hoping that helps?

[georgeh0]

Thanks a lot for contributing, @banrovegrie. Will make a new release soon on this new feature.