[lib] DMOperationSpec for join thread operation
Summary:
Create the spec.
https://linear.app/comm/issue/ENG-8741/create-dmoperations-for-jointhreadmessageinfo
Depends on D12835
Test Plan: Just the Flow. Further testing will be performed after it is connected with the rest of the app.
Reviewers: kamil, inka
Reviewed By: inka
Subscribers: ashoat
Differential Revision: https://phab.comm.dev/D12870