[lib] Include Farcaster username in notifs

Summary:
This makes sure that when somebody's Farcaster friend signs up for Comm, they get a notif with their Farcaster username and Comm username.

Depends on D11888

Test Plan: I tested notifs generated on the keyserver and delivered to an Android device, and confirmed that they resolved Farcaster usernames properly

Reviewers: ginsu

Reviewed By: ginsu

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D11889