Add getJobs() method to database backend

I redid this for rc7 using the 'SupportsListingJobsInterface'

In addition to @bojanz' comment, there is a reason why this is usually not exposed by queue servers, and that is because this is something which is intrinsically prone to race conditions: with a worker potentially going on in the background, or a cron being run, and new jobs being added by other workers, the list you obtain has a good chance of being obsolete by the time your are done handling it; hence the limited use as a metric in observability tools or a hint in admin tools, not a feature meant for queue consumers.

There is a SupportsListingJobsInterface which currently has no methods.