[shared] [1/n] Move generic websocket messages in tunnelbroker_messages to own crate

Summary:
This moves ConnectionInitializationResponse, ConnectionInitializationStatus, and Heartbeat from tunnelbroker_messages into websocket_messages.

This enables usage in a planned identity_search_messages crate to avoid duplication.

Test Plan: cargo fmt and build each affected crate

Reviewers: varun, bartek, kamil

Reviewed By: bartek, kamil

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D10802