[tunnelbroker-client] Adding of the build.rs cargo file to compile protobuf

Summary:
This diff introduces adding build.rs file with the Tunnelbroker's protobuf file compilation inside.
Generated files will be used in the following gRPC Tonic client from D6281 and beyond.

Linear task: ENG-2728

Test Plan:
To run build and test patch to the D6281 and run nix develop --accept-flake-config -c bash -c "cargo build".
The expected result is that the library will be successfully built.

Reviewers: jon, bartek, varun

Reviewed By: jon, bartek, varun

Subscribers: ashoat, tomek, atul

Differential Revision: https://phab.comm.dev/D6279