Include sub-group projects in Group Code Coverage feature set

Release notes

Problem to solve

As a Director of App Dev, I want to see coverage data from projects that belong to sub-groups of my group, so I can get a true sense of TOTAL test coverage in the group.

Proposal

• Add to the query that sub-groups are parsed and data sent back in the GraphQL API
• If this is deemed not performant (takes 2x the current time on gitlab.org with all of its sub-projects maybe?) let's talk about making this an option for the user in the interface.
• Graph should now calculate average of all coverage jobs for group and sub-group projects with coverage
• Table should display data from group and sub-group projects (UX we might think about how to display this, maybe just a * on the project name or something?)
• The .csv should include the sub-group project data (Same note as item above about including a way to indicate sub-group projects