[rag evals][3/n] add ability to eval retrieval + generation in agentic eval pipeline

[yanxi0830]

What does this PR do?

• This PR adds the ability s.t. users can evaluate on both retrieval + generation separately & as a whole by passing an AgentConfig to the /eval API
• The memory_retrieval context will be stored in the "context" column used for scoring functions that can evaluate the retrieved context.

Test Plan

• E2E Test RAG Agent Notebook: https://gist.github.com/yanxi0830/0377594d29958f9b6f9317ab049fa836

Sources

Please link relevant resources if necessary.

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
• Ran pre-commit to handle lint / formatting issues.
• Read the contributor guideline,
  Pull Request section?
• Updated relevant documentation.
• Wrote necessary unit or integration tests.