[lib] Partially update UpdateRelationshipMessageSpec to support updated UpdateRelationship message type

Summary:
Update UpdateRelationshipMessageSpec type and make necessary changes to resolve flow issues.

Will update implementation of remaining functions to support UpdateRelationship in followup diffs.

Based on following from hackathon branch: https://github.com/CommE2E/comm/commit/c8ed27f5c5a4b23bd181d562741959140a9b473c

Depends on D11443

Test Plan: flow + close reading + logged return values of each of the functions during the hackathon

Reviewers: varun, tomek, ginsu, will

Reviewed By: ginsu

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D11452