Add GraphQL query for container virtual registry upstream details

✅ What does this MR do and why?

This MR adds GraphQL query for Virtual Registry Container upstreams under the given group to provide a GraphQL equivalent

• Add virtualRegistriesContainerUpstreams query type

  query {
    virtualRegistriesContainerUpstream(id: "gid://gitlab/VirtualRegistries::Container::Upstream/6") {
      id
      url
      cacheValidityHours
      username
      name
      description
      registriesCount
      registryUpstreams {
        position
        registry {
          id
        }
      }
    }
  }

• Add Types::VirtualRegistries::Container::UpstreamDetailsType

• Add virtual_registries_container_upstream in Types::QueryType

References

N/A

Screenshots or screen recordings

N/A

How to set up and validate locally

1. Switch to this branch 579691-add-graphql-container-virtual-registry-upstream-details-type
2. Enable feature flag and setup Virtual registry settings

   # Enable container virtual registry
   ::Feature.enable(:container_virtual_registries)
   
   # Create virtual registry setting
   group = Group.find_by(path: 'gitlab-org')
   user = User.find(1)
   VirtualRegistries::Settings::CreateOrUpdateService.new(
     group: group,
     current_user: user,
     params: { enabled: true }
   ).execute

3. Create virtual registries container upstreams

   group = Group.find 24
   registry = FactoryBot.create(:virtual_registries_container_registry, group: group)
   upstream = FactoryBot.create(:virtual_registries_container_upstream, registries: [registry], name: 'upstream1')
   gid = upstream.to_gid.to_s

4. Go to Graphql explorer (http://gdk.test:3000/-/graphql-explorer) and test the new query type

    query {
      virtualRegistriesContainerUpstream(id: "gid://gitlab/VirtualRegistries::Container::Upstream/6") { # replace gid you got above
        id
        url
        cacheValidityHours
        username
        name
        description
        registriesCount
        registryUpstreams {
          position
          registry {
            id
          }
        }
      }
    }

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 #579691