[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