VSD Usage overview panel - display usage overview panel for subgroups

Problem

The Usage overview panel should be displayed also for subgroups.

Proposal

• Support subgroups for the usage overview
  • Change rootNamespace to namespace in the data source (see #454448 (comment 1864318696))
  • Change rootNamespaceName to namespaceName in the default VSD config
  • Update any related specs
• Project-level is out of scope from this issue.

Based on this discussion.