Turning RAG into a runnable function

[wpfl-dbt]

Context

Testing, changing and swapping our chains is a nightmare. This splits a basic RAG chain out and adds tests to show it works. This lays better ground to improve and extend.

⚠️ Important details

• I use a lower-level class for making the runnable that means we need to write Elastic queries ourselves
• One reason is that I want to return Chunks, not Documents, and keep everything in our domain

Changes proposed in this pull request

• Retriever runnable
• RAG runnable
• Tests for both

Guidance to review

• Run the unit tests
• Ensure you're happy with my decision to use a lower level class for retrieval

Things to check

• I have added any new ENV vars in all deployed environments
• I have tested any code added or changed
• I have run integration tests