GraphQL: Add analytics visualizations for groups

What does this MR do and why?

Allow to query dashboard visualizations in a group level using GraphQL. This mimics the behavior of Project.customizableDashboardVisualizations

part of #414471 (closed)

How to set up and validate locally

1. Make sure to have an ultimate license
2. Enable feature flag: Feature.enable(:product_analytics_dashboards)
3. Create a project that has dashboards configuration.
4. Create a group and assign the project with dashboards configuration to it following these instructions.
5. Execute the following GraphQL query:

 query groupVisualizations{
   group(fullPath: "Flightjs") {
    name
    customizableDashboardVisualizations {
      nodes {
        data
        options
        errors
      }
    }
  }
}

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.