[native][web] Allow sending custom encrypted message via Tunnelbroker test interface

Summary:
Allowing this interface to send arbitrary messages helped me test DM ops in ENG-8426. I figured this might be useful in the future, and potentially to other memebers of the team.

Depends on D12825

Test Plan: I followed the test plan described in this Linear comment, which relies on these changes to allow manually sending encrypted messages between clients

Reviewers: tomek, kamil, marcin

Reviewed By: tomek

Differential Revision: https://phab.comm.dev/D12826