[lib] Hook everything together in FarcasterDataHandler

Summary: This component handles fetching a user's Farcaster friends via Neynar, mapping them to Comm user IDs via the identity service, and then starting threads with them on the authoritative keyserver.

Test Plan: Testing this during the Hackathon in combination with other diffs. Also tested it later at the office with Ginsu and Atul

Reviewers: varun, ginsu, atul

Reviewed By: ginsu

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D11460