[lib] implement adding thread members for DMs

Summary:
ENG-8568

This hook which wraps two needed operations together, add_viewer_to_thread_members and add_members.

Doing this in parallel, not at once is bad, created a task to fix this: ENG-9103, as this is complex change.

Depends on D13182, D13193

Test Plan: Tested later in the stack

Reviewers: tomek, inka

Reviewed By: tomek

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D13197