[lib] DMOperationSpec for add members operation

Summary:
Create the spec.

https://linear.app/comm/issue/ENG-8735/create-dmoperations-for-addmembersmessageinfo

Depends on D12807, D12914

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/D12835