[tunnelbroker] Add RPC for closing WebSocket sessions

Summary:
Address ENG-8217.

Added Tunnelbroker RPC that force-closes WebSocket session with given device ID (if present).
The message is not persisted - it makes no point of storing it if the device isn't connected to Tunnelbroker at the time.

Depends on D13227

Test Plan: Repeated test plan from D13227, now by calling the RPC

Reviewers: kamil, ashoat

Reviewed By: kamil, ashoat

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D13228