[services] Tunnelbroker - Renaming Rust library directory from rust-notifications to rust-lib

Summary:
This diff renaming the Rust code directory from rust-notifications to rust-lib name, because we are going to use a single Rust library with the submodules instead of building separate Rust libraries.

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/D5261