Allow remote controlling the simulation speed via control protocol

[andreaswendler]

Is your feature request related to a problem? Please describe.
At least in our AI development, it is often helpful or necessary to run the simulation at a slower speed or start and stop it rapidly.
While this can be done in grSim already via the UI, this is not sufficient as our framework also needs to be told the simulation speed for the tracking etc. to work properly.

Describe the solution you'd like
A remote control protocol with which the simulation speed can be adjusted. It would be defined as a protobuf message and sent in a similar manner as the robot movement control.

Describe alternatives you've considered
I can not think of an alternate way to achieve this goal.

Additional context
I will likely develop this either way for our internal use since we will not be able to properly work with grSim without it.
However, if possible I would prefer for the feature to be integrated in the official grSim version (both for easier maintenance and since other teams will also be able to use it).