Add coverage tracking and enforcement

What does this MR do and why?

This adds coverage tracking to the specs job and enforcement with a new undercoverage job.

Action items

• (If applicable) Add documentation to the handbook pages for Triage Operations =>
• (If applicable) Identify the affected groups and how to communicate to them:
  • /cc @person_or_group =>
  • Relevant Slack channels =>
  • Engineering week-in-review

Closes #884 (closed).