Feature/custom rag server

[rusenask]

Enables custom RAG servers for Helix apps. Added example nodejs app:

name: custom-rag-knowledge
description: |
  A simple app that demonstrates how to setup Helix with knowledge from a
  custom RAG server
assistants:
- name: Helix
  description: Knowledge about cars
  knowledge:
  - name: cars
    rag_settings:
      disable_chunking: true # Pass the entire document to the RAG server
      index_url: http://1rvyoove1fvtgxtm3t38yf.webrelay.io/api/index
      query_url: http://1rvyoove1fvtgxtm3t38yf.webrelay.io/api/query
    source:
      web:
        urls:
        - https://gist.githubusercontent.com/rusenask/d7d12da5bf8dd11a512e2f8143a4bd84/raw/bbf65a70aad34057b5595cb2aeaa8cf0c7d0277d/cars