UX: Link existing upstream flow in Maven Virtual Registries

Problem

While working on #548558 to add support for local upstreams, we realised that the "Link existing upstream" flow for Maven Virtual Registries hasn't been designed yet in #493842 (closed).

Currently, users can create new external upstreams, but if they want to reuse an upstream that was already created, the experience needs a dedicated UI. This issue introduces the design for that flow.

Solution

The goal is to allow users to reuse an existing external upstream by linking it to a Maven Virtual Registry.

Th solution should provide a clear and efficient way for users to link an existing external upstream to a virtual registry without re-entering details. The solution includes:

• A dedicated page for linking an existing upstream.
• An upstream selector with search and description display.
• Read-only summary of the selected upstream’s details.
• Display of allow and deny patterns associated with the upstream.
• Option to test upstream connection and URL against allow/deny patterns.