Add productAnalyticsDashboards GraphQL type to groups

Problem(s) to solve

What data to put into a group level product analytics dashboard

As a team lead/product manager with features in multiple GitLab Projects, I want to see a unified view of feature usage in one spot, so I can make decisions about what to work on next.

Some use cases may include (need validation)

• display saved visualizations from multiple projects in one dashboard
• Aggregate data from multiple projects (page views, events, etc.) into a single group level visualization.

Display of multiple group level dashboard types

For the group-level Analytics dashboard listing we need the backend to get a list of builtin & custom dashboards. Without this feature users won't be able to create & read builtin nor custom dashboards from files and dashboards will have to be hardcoded.

Note that we have an issue to Rename Product Analytics GraphQL types to Analy... (#412290 - closed).

Potential solutions

• Group owners set an analytics project to read & write custom dashboard files. Similar to how projects are set up. Additionally:
  • Add a group-level GraphQL API that mimics Project.productAnalyticsDashboards.
  • Add a group-level GraphQL API that mimics Project.productAnalyticsVisualizations.
• Groups return a list of dashboards in each project with analytics enabled.