Add CompleteTime to the Jobs API

Currently, jobs have a `SubmitTime` field but no equivalent field for when a job transitions into a failed/complete/lost state.

The closest alternative is `ModifyTime` on allocations and `FinishedAt` on tasks within the allocations.

By bubbling up, say, the last `FinishedAt` timestamp for the last task in the last allocation to the job record itself, we'd save on API traversals when trying to compute job duration.

The clearest use case here would be to get the duration for batch jobs, particularly periodic and parameterized ones. Related issue here #4263