Feature request

Feature description

rclpy follow-up of ros2/rclcpp#1542.

the current implementation of simulation time which listen to the /clock topic, this behavior breaks the symmetry between the usage of real time (system time internally accessed without the need of subscribing to a topic, and hence always updated) and simulation time (time received on the /clock topic and hence frozen in a callback).
the symmetry should be maintain in order to not draw false conclusions about the behavior of a code developed in simulation VS real time, and thus independently from good/bad code practice.

Implementation considerations

N.A