This diff introduces adding of the `shared/tunnelbroker-client/target` folder to the global `.gitignore` file.
The reason to add this folder is that the client shared library can be built and tested in an isolated way and will produce temporary build files during the Cargo building and testing process using `cargo test` or `cargo build`.
Linear task: [[ https://linear.app/comm/issue/ENG-2728/implement-tunnelbroker-shared-client-library | ENG-2728 ]]