[lib] Introduce FarcasterRelationshipRequest, validators, and unit tests

Summary:
Introduce FarcasterRelationshipRequest which will take a dictionary of commUserID -> farcasterUserID entries instead of array of userIDs like the "Traditional" RelationshipRequests.

Depends on D11592

Test Plan: flow and unit tests for validator passes

Reviewers: ginsu, tomek, will, varun

Reviewed By: tomek

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D11593