[services/commtest] Tunnelbroker - Adding of helper function for the string signing with the private key

Summary:
This diff introduces the function to make a signature for the string by the private key. The signature is needed to prove that the public key provided to the Tunnelbroker server belongs to the user during the session creation call.
This function is called during the session creating a call in D5931.

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

Reviewers: jon, bartek

Reviewed By: jon

Subscribers: ashoat, tomek, atul

Differential Revision: https://phab.comm.dev/D5927