[lib] Introduce bare minimum messageTypes.UPDATE_RELATIONSHIP

Summary:
Basically fill out update-relationships with contents of legacy-update-relationships with additional farcaster_mutual operation.

Based on following commit from hackathon branch: https://github.com/CommE2E/comm/commit/8dd3e3339b8ac1435bc79064c2a9253bb07334e8

Depends on D11436

Test Plan: flow/things work as expected in subsequent diffs

Reviewers: ginsu, varun, tomek, will

Reviewed By: ginsu

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D11438