feat: Add support for PostgreSQL vector store

[cvauclair]

Feature Request

Add vector store support for PostgreSQL (with pg_vector extension).

Motivation

PostgreSQL is one of the most widely used open source databases and supports vector search via the pg_vector extension. This makes the database a good integration candidate for Rig.

Proposal

• Create a new companion crate in the repository called rig-postgres.
• Implement the VectorStore and VectorStoreIndex traits for the selected PostgreSQL client.

Note: The integration should be similar to the MongoDB vector store integration (see rig-mongodb).