Add Entity for default_queue table

I think it would be nice to have entity beside the SQL file, so that we can use this in combination with "orm:schema-tool:update" instead of manually importing SQL. Please review.
yes, doctrine has such a feature. However for the current usage we expect the attribute to be string no. It is serialized on our end anyway... https://github.com/juriansluiman/SlmQueueDoctrine/blob/master/src/SlmQueueDoctrine/Queue/DoctrineQueue.php#L76