RPC - Bind Workload Event

Gearman service will throw an event before the job's workload is returned enabling listeners to modify workload content.

Example listener:
```
class GearmanListener 
{
 public function onBindWorkload(BindWorkloadDataEvent $event)
    {
        $job = $event->getJob();

        $this->injectWorkloadEnvironment($job);
    }
}

<service id="hautelook.gearman.listener" class="Hautelook\ApiBundle\EventListener\GearmanListener">
    <argument type="service" id="hautelook_gearman.service.gearman" />
    <tag name="kernel.event_listener" event="gearman.bind.workload" method="onBindWorkload" />
</service>
```

Related - 
https://github.com/bwoodmansee/halo-symfony/commit/32c2e5dd5012c824fabff881f900f662e5ac6663