Support Structured and Multi-Modal Response Types in Bedrock Retriever

[000-000-000-000-000]

Purpose

Adds support for structured and multi-modal retrieval by updating the Bedrock retriever response parsing logic

Detailed Changes

1. Modularize retrieval logic in Bedrock retriever to separate a/ request creation b/ response parsing c/ search result to document conversion
2. Make the retrieval config clearly optional + make it clear that it can be either a dictionary or a Python object
3. Add TEXT, IMAGE, and ROW parsing for search results
4. Introduce broader testing regime to test more cases across all of the different output types

Testing

1. Added and executed new unit tests
2. Installed and ran manual local tests on a few existing Knowledge Bases