[services] Tunnelbroker - Modify server handler to use Rust signature verification

Summary:
This diff modifies the Tunnelbroker new session gRPC server handler to drop using the C++ cryptopp function for the signed string verification in a favor of using the Rust verify_signed_string function from D5945.

Removing deprecated code from the C++ codebase is in the following D5948.

Linear task: ENG-2492

Test Plan:
Testing process:

• 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, tomek

Reviewed By: jon, tomek

Subscribers: ashoat, tomek, atul

Differential Revision: https://phab.comm.dev/D5947