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

Summary:
This diff introduces adding the helper method to the Commtest integration API tests with the gRPC client to call the SessionSignature request to the Tunnelbroker server and return a result from a call.
This function is called in D5931 where the tests themselves are called.

Linear task: ENG-1657

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

Reviewers: jon, bartek

Reviewed By: jon, bartek

Subscribers: ashoat, tomek, atul

Differential Revision: https://phab.comm.dev/D5837