[lib] Modify add viewer to members operation so that creating a message is optional

Summary:
When one user's device shares threads with the other, we don't want to create robotext messages.

https://linear.app/comm/issue/ENG-9241/introduce-a-handler-that-sends-operations-to-new-devices

Depends on D13292

Test Plan: Process an operation with and without the message ID and check if the message was created only in the first case.

Reviewers: kamil, inka

Reviewed By: kamil

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D13304