[services] Tunnelbroker - Add FCM crate to Cargo dependencies

Summary:
This diff adds dependencies to the Rust Cargo manifest file for the notifications library wrapper:

• FCM - FCM client library.

The Cargo.lock file is added to lock versions.

Linear task: ENG-1330

Test Plan: Run cargo build from the services/tunnelbroker/rust-notifications directory.

Reviewers: karol, varun, jon, tomek, ashoat

Reviewed By: jon, tomek, ashoat

Subscribers: ashoat, tomek, atul, karol, abosh

Differential Revision: https://phab.comm.dev/D4893