Implement the register function of OC\ContactsManager

Tested this with the following code. The $key parameter is not really used in this implementation, because the keys for the address books are not predictable. Can we modify the interface and drop the key parameter?

```
$cm = \OC::$server->getContactsManager();
$cm->register('contacts', function() use ($api) {
	if (\OCP\User::isLoggedIn()) {
		$app = new App($api->getUserId());
		$addressBooks = $app->getAddressBooksForUser();
		foreach ($addressBooks as $addressBook)  {
			if ($addressBook->isActive()) {
				\OCP\Contacts::registerAddressBook(new AddressbookProvider($addressBook));
			}
		}
	}
});
```

@tanghus @DeepDiver1975