[Tunnelbroker] add WebSocket version of MessageToDevice

Summary:
Adding new type based on this defined in RPC to send messages between devices.

Depends on D9313

Test Plan: cd shared/tunnelbroker_messages && cargo test

Reviewers: michal, bartek, varun

Reviewed By: michal, bartek

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D9314