MultiCameraProvider camera scanning

[NiklasNeugebauer]

This adds the scan_for_cameras method to the MultiCameraProvider which in turn calls it on all its providers.
For this, all CameraProviders need to support the scan_for_cameras method which is now an abstract method of the parent class.

Adding the method as an abstractmethod may require action on all derived CameraProviders outside of rosys.
Another option would be to add it as a default that does nothing.

The central question here would be if all providers can scan for new cameras somehow or if some need a fixed list provided by the application code.