[services] Tunnelbroker - Move ffi from main.rs to the cxx_bridge.rs file

Summary:
This diff moves the FFI functions from CXX from main.rs to the cxx_bridge.rs file.
The reason for moving the FFI away from the main.rs to its own scope is that the ffi::* functions should be called from the gRPC server handlers (for example in D5487).

Linear task: ENG-2094

Test Plan: The service was successfully built with the CI process or can be built manually using the cargo build command in services/tunnelbroker directory.

Reviewers: jon, varun, marcin, tomek

Reviewed By: jon, tomek

Subscribers: ashoat, tomek, atul, abosh

Differential Revision: https://phab.comm.dev/D5486