Resolve "Graph the average code coverage of all of a group's projects"

What does this MR do?

Related to #215140 (closed)

Add a graph to the group repositories analytics page to represent the group projects average test coverage over the past 30 days.

Screenshots (strongly suggested)

group_coverage_graph

When chart is empty

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• [-] Style guides
• [-] Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done