Spring-ai Modular RAG: Retrieval with Vector Stores

• Establish new package for Modular RAG components.
• Add new Query API, representing a query in the context of a RAG flow.
• Define Retrieval package for the RAG building blocks handling the data retrieval operations.
• Relocate DocumentRetriever to Retrieval package and implement VectorStoreDocumentRetriever.
• Introduce RetrievalAugmentationAdvisor as the successor of QuestionAnswerAdvisor. It uses the Retrieval building blocks described in the previous point.
• Make Advisor APIs null-safe and update tests accordingly.

Relates to gh-https://github.com/spring-projects/spring-ai/issues/1603

It might also address the requests from https://github.com/spring-projects/spring-ai/issues/1290

Comment From: markpollack

merged in 5d8c032bb706a04876687cb801f98b3f84417e50