[New feature] Monitorized sequential startup

[OscarMrZ]

In some exercises, mainly follow_person, launching the world and spawning the robot are two separate steps that need to be executed always sequentially. A hotfix has been implemented at issue-366 of RoboticsInfraestructure, and this would be the next step.

I suggest adding a new step in the next version of the manager so the robots can be spawned at any moment providing the scenario is launched. Afaik, this is common practise within the ROS 2 community.