[lib] Fix farcasterRelationshipRequestValidator

Summary:
exactlyTwoDictEntriesValidator is only relevant for UpdateRelationshipMessages, not for the request. Multiple farcaster mutuals can be included in single FarcasterRelationshipRequest.

Depends on D11612

Test Plan: Able to include multiple users in single request farcaster request (hard-coded dummy FIDs in request)

Reviewers: ginsu, tomek, will, varun

Reviewed By: tomek

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D11624