[ticket/12352] Use custom provider collection for auth providers

Using this custom provider collection, we can properly check whether the
configured auth provider does exist. The method get_provider() has been added
for returning the default auth provider or the standard db auth provider if
the specified one does not exist.
Additionally, the method get_provider() will throw an RuntimeException if
none of the above exist.

PHPBB3-12352

phpBB/config/auth_providers.yml

@@ -1,8 +1,9 @@
 services:
     auth.provider_collection:
-        class: phpbb\di\service_collection
+        class: phpbb\auth\provider_collection
         arguments:
             - @service_container
+            - @config
         tags:
             - { name: service_collection, tag: auth.provider }
     auth.provider.db: