Draft: New metrics for completed and failed jobs

Callback to query the testcases for the failed jobs to try to distinguish error sources in the metrics.

This is the part of a much larger set of changes which is specific to failure tracking. The unit test for this work depends on significant additional changes, and isn't included in this PR: it requires the new lava-api-mock crate and introduces dependencies on boulder and django-query which are still being finished off.

This is draft pending

• Merge of lava-api!15 (merged) [required API]
• Merge of lava-monitor!14 [code is based on this]
• Completion of the current tests against staging