Add PostgreSQL pgvector datastore

[mmmaia]

Closes #36 by adding PostgreSQL pgvector as a datastore option.

• Supports any PostgreSQL standard connection (self-hosted or hosted providers)
• Uses SQAlchemy for abstraction
• Uses a JSONB field for metadata
• Do not create indexes on table setup, per Impact of Indexes on Query Result Sets pgvector/pgvector#77
• Implements integration tests
• End to end tested with a ChatGPT Plugin loaded with OpenAI's documentation

[isafulf]

This is awesome, thanks for adding! Left some comments.

A few things:

• Could you make sure that the functionality of the implementation of the datastore functions is consistent with the other implementations (specifically, supporting querying with filters and deleting with filters)?
• Also, some more detail in the setup instructions in the readme would be great.
• Have you run this to verify that your implementation works?
• Could you consider adding integration tests, similar to the integration tests for the other vector dbs in the tests folder?
• I have referred you as a tester for developer plugins, once you are off the waitlist it would be great if you could test the retrieval plugin with pgvector end to end.

To anyone else reading this -- feel free to contribute to the PR.

[mmmaia]

Thanks for the review @isafulf!
I addressed your comments and will run an end-to-end test of the plugin soon.