Setup route for virtual registries

What does this MR do and why?

• Adds virtual registries index route at /groups/<group-id>/-/virtual_registries. Route should be available
  • only for root groups
  • only when feature flags virtual_registry_maven and ui_for_virtual_registry are enabled
  • licenced feature packages_virtual_registry is available
  • when Gitlab.config.dependency_proxy.enabled is true
  • has read_virtual_registry permission.
• Adds controller, specs
• Renders index template with page title
• Adds virtual_registry_policy_subject method to group model
• Introduce ui_for_virtual_registries feature flag

EE: true

References

• Sidebar entry will be added in a separate MR after approval Navigation change proposal: Sidebar navigation ... (#525926 - closed)
• Design: #493842[Virtual-registries-landing.png]

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Make sure your GDK has an enterprise licence: https://gitlab.com/gitlab-org/gitlab-development-kit/blob/main/doc/index.md#use-gitlab-enterprise-features.
2. Enable feature flags virtual_registry_maven & ui_for_virtual_registry
3. Visit /groups/<group-id>/-/virtual_registries for root group should show above screenshot
4. Visit /groups/<group-id>/-/virtual_registries for sub group should render 404 page

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Related to #525927 (closed)