introduce names for user backends - via IUserBackend

cc @DeepDiver1975 @icewind1991 @PVince81 @DeepDiver1975 

Is this how we should add stuff to a interface that is implemented by third party apps?

ref #12620 #12806 
This one is a bit older ;) It's already in stable5 ;) https://github.com/owncloud/core/blob/stable5/lib/public/userinterface.php