[lib] Add support to useUpdateRelationships for sending relationship robotext to thick threads

Summary:
This diff addresses ENG-9330.

Some complexity was necessary to allow us to fetch device lists for the new user prior to sending the message. More context on that complexity in this comment.

Depends on D13441

Test Plan: I tested all three cases of RobotextPlanForUser. I tested on both iOS and Android, with two clients talking to each other. I sent a friend request, accepted it, then removed the relationship and did the flow again.

Reviewers: tomek, kamil

Reviewed By: tomek

Subscribers: kamil

Differential Revision: https://phab.comm.dev/D13443