[5.5] Ability to determine if queued handler should be pushed to queue

[themsaid]

An event that has multiple queued handlers, sometimes a single handler is only desired to run in a certain condition, normally you'd let the handler be queued and when workers run it you can just return null in handle() method if it shouldn't be run, but you end up with filling your queues with unnecessary jobs.

This PR adds the ability to add a shouldPushToQueue() on the handler class to determine if that queued handler should be pushed to queue or not:

public function shouldPushToQueue($event){
    return $event->user->shouldReceiveOffer();
}

[taylorotwell]

@themsaid can you PR a test for this?