This diff adds a Buildkite CI test for the Tunnelbroker client-shared library.
By running the `cargo build` and `cargo test` from the Nix in our CI we will make sure that the diffs changes are passing the CI test for the Tunnelbroker client library.
Linear task: [[ https://linear.app/comm/issue/ENG-2728/implement-tunnelbroker-shared-client-library | ENG-2728 ]]