sys_time: publish /clock for simulation times

[dayjaby]

I hope ROS1 Pull Requests are still ok :-)

When replaying PX4 log files plus rosbag files, we need a way to sync the times between PX4 ulg and the rosbag. This can be accomplished by the following procedure:

• Set /mavros/time/publish_sim_time to true
• increasing the rate of SYSTEM_TIME being sent to mavros, e.g. by mavlink stream -u 14556 -s SYSTEM_TIME -r 100
• Set /use_sim_time to true
• Run rosbag play with --clock argument

Alternative: Don't invent a new parameter, but rely on /use_sim_time instead.

[vooon]

LGTM, Thanks!

I think in that case extra parameter is preferable as it gives you more flexibility for debugging.