[Tunnelbroker] Update example tunnelbroker.ini

Summary:
Add missing notification settings which are needed by tunnelbroker

https://linear.app/comm/issue/ENG-3670

Test Plan:

cd services/tunnelbroker
cp tunnelbroker ~/.config/

cargo run # doesn't immediate fail on trying to parse values

Reviewers: varun, bartek, max

Reviewed By: varun

Subscribers: ashoat, tomek, atul

Differential Revision: https://phab.comm.dev/D7391