[v3.0.0bx] Make vector store configurable

[dlqqq]

Problem

Users would like to use alternative retrievers for RAG instead of being restricted to using FAISS locally.

Proposed Solution

1. Offer alternative retrievers, such as Pinecone, Kendra, OpenSearch, etc.
2. Offer retriever configurability to allow operators to switch between them.

Additional context

Not sure if retriever configurability should be exclusively implemented via traitlets; I would prefer an admin UI with authorization and the ability to change the retriever at runtime (instead of prior to server start), but I'm not sure if this is possible within our time constraints.