introduce names for user backends - via IUserInterfacev2

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 
Okay... UserInterface... I thought it was about the web UI but actually managing users.
Should probably be called IUserManagement or something else instead. Especially that the "I" is already there for "Interface"