[tunnelbroker] Make WebSocketSession generic

Summary:
We don't rely on specific websocket stream type, so this can be made generic.
This will be used in the next diff, where we will change TcpStream to a different type.

Depends on D9155

Test Plan: Tunnelbroker still builds, integration tests pass.

Reviewers: kamil, michal, varun, jon

Reviewed By: kamil, michal

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D9156