Postgres PGvector retriever hardcodes old embedding model

[josh-mantovani]

The PgVectorRm hardcodes the embedding model to be text-embedding-ada-002. They have new embedding models and it'd be great to allow users to specify this. https://github.com/stanfordnlp/dspy/blob/01455bd20f5770819aff9edc0b43915203f71cbd/dspy/retrieve/pgvector_rm.py