[lib] Split add members spec
Summary:
Split the spec so that we don't send extraneous data to all the existing members.
https://linear.app/comm/issue/ENG-8929/split-add-members-operation
Test Plan:
Checked two scenarios:
- Viewer creates a thread and adds a new user to it using add_members operation
- Viewer is added to a thread using add_viewer_to_thread_members
In both cases the thread appeared in the store with correct membership array.
Reviewers: kamil, inka
Reviewed By: inka
Subscribers: ashoat
Differential Revision: https://phab.comm.dev/D13052