Adds mavenUpstreamRegistry top-level GraphQL query

What does this MR do and why?

Adds mavenUpstreamRegistry top-level GraphQL query to look up Maven upstream registry by its ID.

Adds registries field to maven_upstream_type This will be used in Show associated registries before Maven upstrea... (#556828 - closed) to show associated registry names.

EE: true

References

• Related issue: GraphQL query & mutations for virtual registry ... (#525932)
• Related MR: Adds mavenVirtualRegistry top-level GraphQL query (!199389 - merged)

Screenshots or screen recordings

N/A

How to set up and validate locally

1. Run the following in the rails console

Feature.enable(:virtual_registry_maven)
g = Group.find(1) // must be a root group
r = VirtualRegistries::Packages::Maven::Registry.create(group: g, name: 'app-test') 
# create an upstream, note: if you get validation error for local URL you need to comment out url validations in ee/app/models/virtual_registries/packages/maven/upstream.rb
u = VirtualRegistries::Packages::Maven::Upstream.create!(group: g, url: 'http://local.test/maven/', name: 'Upstream')

# create a registry_upstream join table row
VirtualRegistries::Packages::Maven::RegistryUpstream.create!(group: g, registry: r, upstream: u)

2. Run the example query

query {
  mavenUpstreamRegistry(id: "gid://gitlab/VirtualRegistries::Packages::Maven::Upstream/<u.id>") {
    id
    name
    description
    url
    username
    cacheValidityHours
    metadataCacheValidityHours
    registries(first: 20) {
      nodes {
        id
        name
        description
        updatedAt
      }
    }
    registryUpstreams {
      id
      position
    }
  }
}

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Related to #556828 (closed)