Plugins for additional sensor models?

[AravindaDP]

I would suggest adding a plugin support for additional stage sensor model controllers.
I think this can simply be implemented by defining a plugin API with int SubscribeModels() and void WorldCallback() (and any other required functions) and calling those API methods from respective functions in StageNode class.
This way we should be able to add support for additional sensors like blobfinder and fudicial sensor etc from ROS side as well.
If we can agree upon the proposed changes I would be more than happy to contribute in the development.