[lib] Show Farcaster username in farcaster_mutual robotext

Summary:
This lets us show the Farcaster username of the friend that was automatically added via the farcaster_mutual message.

Depends on D11882

Test Plan: I tested resolution of Farcaster usernames in robotext. I tested iOS, Android, and web. I tested from both the sender and recipient's perspective

Reviewers: ginsu

Reviewed By: ginsu

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D11888