[ticket/12775] Add a container_factory class and remove functions_container

https://tracker.phpbb.com/browse/PHPBB3-12775
https://tracker.phpbb.com/browse/PHPBB3-12420
https://tracker.phpbb.com/browse/PHPBB3-12774
(Useful for https://tracker.phpbb.com/browse/PHPBB3-12685)

PHPBB3-12775
config.php is always constructed so we can define the service as synthetic (and we can inject the object).
We could do the same for the DB but in this case we need to always call get_dbal_connection() (even if we don't use the extensions)