Implement the Worker part of the Product Construction Service

[dkurepa]

Since the Product Construction Service will work on a workitem queue system, we'll need something to process those workitems.
We need to keep in mind that it's highly likely that we'll have multiple replicas running at the same time, all consuming these workitems.
An important piece of this work items are the deployments. We want to be able to notify all workers that a deployment is about to happen, have them finish their current workitems, notify the deployment script/service, and then stop, continuing only after the deployment, as new revisions.

[dkurepa]

The PR adding the job processor has been merged. For now it's just has a basic job type, that prints out the text. We will keep adding new jobs/JobProcessors as we need

[dkurepa]

The second part of this issue is in #3260. This is the part that deals with the deployments

[dkurepa]

I created a second issue for the deployment part of this issue #3269. Closing this now