Adds virtual registries index and new routes by registry type

What does this MR do and why?

Adds virtual registries index and new registry type routes

Namespace virtual registries routes to groups/virtual_registries directory

Rename virtual_registries_controller to virtual_registries/registries_controller

Adds controller & templates for index_by_registry_path & new routes

References

• Previous MR to setup index route: Setup route for virtual registries (!185201 - merged) • Rahul Chanila • 17.11
• Based on discussion, we want to identify virtual registries route with registry type
• Designs: Design discussion: Virtual registries (#493842 - closed)

Screenshots or screen recordings

Index maven route	New maven route

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 render page with title Virtual registries
4. Visit /groups/<group-id>/-/virtual_registries/maven for root group should render page with above screenshot
5. Visit /groups/<group-id>/-/virtual_registries/maven/new for root group should render page with above screenshot
6. Visit /groups/<group-id>/-/virtual_registries for sub group should render 404 page
7. Visit /groups/<group-id>/-/virtual_registries/invalid for root group should render 404 page
8. Visit /groups/<group-id>/-/virtual_registries/invalid/new for root group should render 404 page

MR acceptance checklist

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Related to #525927 (closed)