Add localusers contacts to core

In this PR the currently implemented Localusers backend for the contacts app will be moved to core.

@DeepDiver1975 is this how I should implement it? The `\OC\Contacts\Backend\LocalUsers` class will be almost the same as `\OCA\Contacts\Backend\Localusers`. (which will be removed of course)

Some questions:
 - every contacts backend extends the `\OCA\Contacts\Backend\AbstractBackend` class. This should stay the same for the localusers class in core. Should I move the `AbstractBackend` class to the public namespace of core, and remove it out of the contacts app?
 - To update the DB with the contacts should I use hooks on user regristration, deleteing etc, or check if something has changed every time the contacts are fetched. (This is the current implementation.) 
 - where should the DB schemes go?