[Tunnelbroker] Create session creation and key messages

Summary:
Remove "legacy" tunnelbroker api messages, and replace with
websocket messages.

Part of https://linear.app/comm/issue/ENG-3676

Depends on D7431

Test Plan:
This is more intended to be a design diff. However, deserialization is tested

cd shared/tunnelbroker_messages
cargo test

Reviewers: ashoat, varun, bartek, max

Reviewed By: bartek

Subscribers: tomek, atul

Differential Revision: https://phab.comm.dev/D7480