Add support for different knowledge retrieval methods

This is for the built-in retrieval tool.

Currently, the current knowledge retrieval implementation uses a very naive retrieval which simply returns the full contents of every attached file (source).

The current implementation also only support text file types like text/plain and markdown, as no preprocessing or conversions are done at the moment.

It shouldn't be too hard to add support for more legit knowledge retrieval approaches, which would require:

• processForFileAssistant - File ingestion pre-processing for files marked with purpose: 'assistants'

  • converting non-text files to a common format like markdown (this is probably the hardest step to do well across all of the most common file types)
  • chunking files
  • embedding chunks
  • storing embeddings to an external vector store; make sure to store the file_id each chunk comes from for filtering purposes

• retrievalTool - Performs knowledge retrieval for a given query on a set of file_ids for RAG.

  • embed query
  • semantic search over vector store filtering by the given file_ids

Integrations here with LangChain and/or LlamaIndex would be great for their flexibility, but we could also KISS and roll out own using https://github.com/dexaai/dexter