New Satellite Command get_run_time

This MR implements a new standard command for satellites which returns the starting time of the current or last run.

With this information a newly joining controller can figure out how long a run has been active, an information otherwise lost when closing the connection. I decided for a time point of the start because a) that's static and less stuff to do on the sending side and b) it is a native MsgPack type. Only downside is of course differences in system clock.

Python implementation missing @hanno.perrey