[BUGFIX] All documents returned for the summariser

[wpfl-dbt]

Context

The all document retriever wasn't bringing back all documents -- it was limited to 10. Fixed with a new class, which in turn throw up some more problems that needed fixing.

Changes proposed in this pull request

• Adds AllElasticsearchRetriever to return all chunks in order
  • Removed embeddings from the returned object
  • Implemented scan() to get all results
• Enforces the shape of returned documents at the retrieval class level, and simplifies the returned structure
  • Chunk and Document both handled
• Refactors core_api.src.retriever to be simpler for the new classes and functions\
• Refactors API to use these changes

Guidance to review

• Look through core_api.src.retriever most closely -- this is where the action is
• Ensure it all runs

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

[jamesrichards4]

Looks great, this is what my refactor should have been in the first place