[services] Tunnelbroker - Move Rust notifications library code to the submodule

Summary:
This diff moves the Rust notifications code into the notifications submodule as a part of the transition to using a single Rust library linked to the C++ codebase.

This is a part of the stack.
The full context is available in the Linear task.

Related Linear task: ENG-1895

Test Plan: Service is successfully built and the Rust function is successfully called from the C++ side.

Reviewers: jon, michal, marcin, varun, tomek

Reviewed By: jon, marcin, tomek

Subscribers: ashoat, tomek, atul, abosh

Differential Revision: https://phab.comm.dev/D5250