API: Add helpful information to job_statuses output to improve monitoring

Using https://xxx.zendesk.com/api/v2/job_statuses.json I realised that there is a "Completed at" message, but no "Submitted at" message or "runtime" information that shows the duration of the processed job.

This would allow to monitor how long jobs take and if there are changes in runtime (due to high load or error), to act accordingly (e. g. limit the amount of jobs in peak times).

I hope that's an easy fix, it will help devs that make use of the API.

Best regards,
Oliver