This diff introduces adding the helper method to the Commtest integration API tests with the gRPC client to call the `SessionSignature` [[ https://github.com/CommE2E/comm/blob/40eb1354aa060a1c82282a21d89d8eb2a40bc3ab/shared/protos/tunnelbroker.proto#L8 | request ]] to the Tunnelbroker server.
Linear task: [[ https://linear.app/comm/issue/ENG-1657/[new-tests]-add-grpc-integration-tests-for-tunnelbroker-service-to | ENG-1657 ]]