[lib] Split UpdateRelationshipMessageData into UpdateTraditionalMessageData and UpdateFarcasterMessageData

Summary:
Pretty straightforward, the UpdateFarcasterMessageData variant includes creatorFID and targetFID.

Considered something like

+userIDsToFID: { +[userID: string]: string },

for consistency with FarcasterRelationshipRequest, but think this approach is cleaner and we don't need validators/etc to ensure that there are only two entries in userIDsToFID, etc.

Depends on D11624

Test Plan: flow + CI

Reviewers: ginsu, tomek, varun, will

Reviewed By: tomek

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D11641