[services] Tunnelbroker - Adding signed string verification function

Summary:
This diff introduces a signed string verification function verify_signed_string to use in the "new session" creation process in the Tunnelbroker API. This function will be used in the following D5947 to verify the signed string by the public key provided to make sure the client "own" keys.

Linear task: ENG-2492

Test Plan:
As this diff is a part of the stack this diff can be tested on the end of the stack at D5947.
To test it:

• Patch to the D5947
• Run the integration tests in D5931 using the cargo t --test tunnelbroker_integration_test command.

The expected result is passing the integration test (signing and verifying the string are successful and the new session creation test will pass).

Reviewers: jon, bartek

Reviewed By: jon

Subscribers: ashoat, tomek, atul

Differential Revision: https://phab.comm.dev/D5945