[Tunnelbroker] add MessageReceiveConfirmation struct

Summary:
We need this struct to confirm that the message was processed by the client.

Depends on D9574

Test Plan: Deserialization tests, actual usage in next diff

Reviewers: michal, bartek, varun

Reviewed By: bartek

Subscribers: ashoat, tomek, wyilio

Differential Revision: https://phab.comm.dev/D9575