[lib] Split RelationshipAction and RelationshipActionSansFarcaster

Summary:
We want RelationshipActionSansFarcaster so we can modify RelationshipRequest to be a union of something like

{action: RelationshipActionSansFarcaster, userIDs: string[]} | {action: 'farcaster', userIDsToFIDs: {[userID: string]: fID}}

This diff doesn't do much on its own, but want to put up simple changes separately from the maybe more "opinionated" changes?

Test Plan: yarn flow:all

Reviewers: ginsu, tomek, will, varun

Reviewed By: tomek

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D11592