[Tunnelbroker][lib] add FCM notif message type

Summary:
Message send from client to send notif using FCM service.

Depends on D12769

Test Plan: Tested in D12772.

Reviewers: bartek, marcin

Reviewed By: bartek, marcin

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D12770