parameter on_set callback

[dustingooding]

With the demise of Dynamic Reconfigure, rtt_ros2_params should expose the rclcpp::Node::add_on_set_parameters_callback interface (https://docs.ros2.org/foxy/api/rclcpp/classrclcpp_1_1Node.html#a12d535bced9f26b65c0a450e6f40aff8). This will allow for callback-based property updates.