[services/commtest] Tunnelbroker - Adding the tonic client builder function

Summary:
This diff introduces a function to build a Tonic client for further API calls in integrational tests. The pointer to the client will be passed to the helper functions (in D5931) instead of making the "global" client instance.

We are using COMM_SERVICES_PORT_TUNNELBROKER environment variable to provide a port to connect to the Tunnelbroker.
The COMM_SERVICES_HOST_TUNNELBROKER environment variable provides a host to connect to the Tunnelbroker.

Linear task: ENG-1657

Test Plan: The commtest app is successfully built and the tests are successfully passed in the following D5931 using this function results.

Reviewers: jon, bartek

Reviewed By: jon, bartek

Subscribers: ashoat, tomek, atul

Differential Revision: https://phab.comm.dev/D5928