[Tunnelbroker] add ConnectionInitializationResponse struct

Summary:
Currently, Tunnelbroker does not respond to whether the session is properly created or not - clients need to assume that is the socket wasn't closed everything is fine.

I think adding this response is convenient and was implemented in the C++ version.

Depends on D9578

Test Plan: Serialization tests

Reviewers: michal, bartek, varun

Reviewed By: bartek

Subscribers: ashoat, tomek, wyilio

Differential Revision: https://phab.comm.dev/D9593