[services/commtest] Tunnelbroker - Adding of helper function for the NewSession client request

Summary:
This diff introduces adding the helper method to the Commtest integration API tests with the gRPC client to call the NewSession request to the Tunnelbroker server.
This function is called in D5931.

Linear task: ENG-1657

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

Reviewers: jon, marcin

Reviewed By: jon

Subscribers: ashoat, tomek, atul

Differential Revision: https://phab.comm.dev/D5838