[lib] add option to mark DMOperationSpecification as sendOnly

Summary:
ENG-8568

There are ops like ADD_VIEWER_TO_THREAD_MEMBERS that are only meant to be sent to other peers, and should never be processed on client which is generating this operation.

Depends on D13181

Test Plan: Tested later in the stack

Reviewers: tomek, inka

Reviewed By: tomek

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D13182