This diff introduces the `get_nonce_to_sign` internal function which is a gRPC unary request to the Tunnelbroker server to obtain a [[ https://www.notion.so/commapp/Tunnelbroker-028bdd578e4c487fb570dbb862952ec6#9758ddb84edd4a9f86ad07a9d8a4b39e | nonce string to be signed ]]. The function gets the pointer to the Tonic client and `deviceID` and returns the nonce from the server to be signed with the `sign_string_with_private_key` function from D5999.
Linear task: [[ https://linear.app/comm/issue/ENG-2434/implement-new-session-grpc-methods-in-rust-for-the-native-client | ENG-2434 ]]