Feature: Add graph retriever

[dartpain]

I think we can make upload / training process more advanced, maybe let users choose some "advanced" options on upload in the UI.

This way we can for example prepare data for graph + vector database.
And when user creates a query it will automatically figure out where which retriever type needs to be used for that source. So I guess its a two part issue:

• Edit structure of source docs in general (clean up a bit, and add retriever type field)

• Add new graph retriever, add graph ingest, add UI to choose ingestion method maybe