Update pinecone backend to use new engines & base class

Problem/Motivation

✨ Make Embeddings Engine Pluggable RTBC introduced new pluggable embeddings, and ✨ Add a trait, base class and interface for AI Search API backend engines RTBC introduce a new interface/base class for backends. The existing Pinecone backend should be updated to use those for compatibility and maintainability.

API changes

Refactor to use the base classes and new configurable embeddings. Should ensure the upgrade path preserves existing embeddings.