Add request_backup to MultiCameraProvider

[NiklasNeugebauer]

CameraProvider derives from rosys.persistent_module, so each provider will have a request_backup function.

Here we overwrite that functions, since we really want all cameras to be backed up by their respective providers.

Alternatively, we could iterate over cameras and back them up into rosys.vision.multi_camera_provider.MultiCameraProvider.json.

I think this is less fitting though since the MultiCameraProvider is meant to be a proxy rather than a provider by itself.