Implements maven upstream registries table

What does this MR do and why?

Implements maven upstream registries table

Adds search by upstream name functionality

EE: true

What this MR does not do:

This MR does not implement:

1. Used by registries column. This data is not available on the REST API & we will be moving this over GraphQL query Add group field to query all maven virtual regi... (!205320) once it is merged.
2. Pagination of upstreams
3. Clear cache button
4. Delete upstream dropdown

These will be implemented in follow-up MRs.

References

• Previous MR which adds tabs Adds tabs to maven registries and upstreams page (!205962 - merged)
• Design: #556837[UpstreamsList.png]
• MR to add GraphQL query Add group field to query all maven virtual regi... (!205320)
  • Once the MR is merged, we will use that query instead. It lets us show Used by registries column and show all shared registries in the delete upstream modal.

Screenshots or screen recordings

Before	After

OPTIONAL: For responsive UI changes, you can use the viewport size table below. Delete this table if not needed or delete rows that are not relevant to your changes.

Viewport size	Before	After
xs (<576px)

How to set up and validate locally

1. Ensure you have enabled the following feature flags

$ gdk rails console
Feature.enable(:maven_virtual_registry)
Feature.enable(:ui_for_virtual_registries)

2. Go to top-level group
3. Go to Deploy -> Virtual Registry
4. Create a new maven registry
5. Click on the Add upstream button
6. Confirm that description related to using locally hosted repositories is added for Upstream URL and Password fields.
7. The required fields are name, url. For the URL, you can use https://repo.maven.apache.org/maven2
8. Click on Create upstream
9. The newly created upstream should be visible on the list. Click on pencil icon to edit newly created upstream.
10. Click on the Maven upstreams breadcrumb to see the page mentioned in the screenshot.
11. Confirm that edit and show link work.
12. Confirm that no results component is shown when search returns empty results.

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 #556837