Maven virtual registry Upstreams list view

Context

We added a top-level API Maven virtual registry: Introduce an endpoint t... (#550728 - closed) which allows us to implement Upstreams tab in #493842[Virtual-registries-list-by-type.png]

API

https://docs.gitlab.com/api/maven_virtual_registries/#list-all-upstream-registries-for-a-top-level-group