[tunnelbroker-messages] Add message type for anonymous session

Summary:
Resolves ENG-4694.
Added a counterpart of ConnectionInitializationMessage for unauthenticated devices.

Test Plan: CI, check if everything compiles.

Reviewers: kamil, michal

Reviewed By: kamil

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D10779