[keyserver] Send farcaster_mutual UPDATE_RELATIONSHIP message when establishing Farcaster relationship

Summary:
This is developed from my Hackathon commit 7024301fcda748efefa900bd952cd8e127aac87d, but updated to reflect changes @atul has made to allow us to include FID:

1. Updates to the structure of the UPDATE_RELATIONSHIP message
2. Updates to the FARCASTER_MUTUAL API

Note that I also updated the code to avoid creating KNOW_OF relationships between the viewer and themselves.

Test Plan: I tested this during the Hackathon, but haven't tested the updates since then. Will try to test this in the office this week

Reviewers: atul, ginsu

Reviewed By: atul

Subscribers: tomek, atul

Differential Revision: https://phab.comm.dev/D11707