[tunnelbroker-client] Adding shared/tunnelbroker-client target folder to the .gitignore

Summary:
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: ENG-2728

Test Plan: Running cargo build or cargo test will produce temporary files in the shared/tunnelbroker-client/target directory.

Reviewers: jon, varun, ashoat

Reviewed By: jon, ashoat

Subscribers: ashoat, tomek, atul

Differential Revision: https://phab.comm.dev/D6278