PostgresVectorStore.GetCollection() currently returns the abstraction interface IVectorStoreRecordCollection<TKey, TRecord>; if a user then wishes to use a PG-specific API on the collection, they then must downcast the interface. This makes it awkward to both use the IVectorStore implementation (as a factory for collection instances) and use connector-specific APIs.

We can simply have GetCollection() return a PostgresVectorStoreRecordCollection, and have an explicit interface implementation to implement the required IVectorStore interface method.