Added services for:

• load controller
• unload controller
• switch controllers
• list controllers
• list controller_types
• reload controller libraries

To do that, besides adding all the respective code I've had to perform the following modifications to the existing code:

• Add the double buffered list from ROS1 control to avoid locking the update thread
• Modify the ControllerLoaderInterface to add the base class name to the constructor
• update() only the active controllers, not all loaded controllers.

The whole resource management implementation is not done yet, since that part is undergoing changes.
It is usually #ifdef'ed out with the tag TODO_IMPLEMENT_RESOURCE_CHECKING