[lib] Introduce relationship callbacks

Summary:
Create a hook that returns functions that modify the relationship. This hook depends only on userID and onErrorCallback so it's a lot more convenient to use.
Depends on D3733

Test Plan: Send friend request by using relationship prompt and check if the state was updated.

Reviewers: benschac, atul, def-au1t

Reviewed By: atul

Subscribers: ashoat, Adrian, karol-bisztyga, yayabosh

Differential Revision: https://phabricator.ashoat.com/D3734