[lib] Implement UpdateRelationshipSpec.shimUnsupportedMessageInfo

Summary:
Shim UpdateRelationship messages with farcaster_mutual operation for now.

Based on the following from hackathon branch: https://github.com/CommE2E/comm/commit/6dcee7ca9c467951a11086fce416c85bb7f3d385

Depends on D11458

Test Plan: Unsupported robotext appears as expected:

Reviewers: ginsu, tomek, varun, will, ashoat

Reviewed By: ashoat

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D11459