[lib] Split change thread settings spec
Summary:
Avoid sending unnecessary data and simplify the spec logic.
https://linear.app/comm/issue/ENG-8929/split-add-members-operation
Depends on D13052
Test Plan:
Checked two scenarios:
- Viewer creates a thread and adds a new user to it using change_thread_settings operation
- Viewer is added to a thread using change_thread_settings_and_add_viewer operation
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/D13053