Draft: Added linkable_upstreams to registry details type

What does this MR do and why?

Adds a new type to the registry details type linkable_upstreams that will only return the upstreams that are linkable to the registry, it will not return any upstreams that have already been linked.

query {
  virtualRegistriesContainerRegistry(id:"gid://gitlab/VirtualRegistries::Container::Registry/44"){
    linkableUpstreams(first:20, search:"name") {
      nodes {
        id
        name
      }
    }
  }
}

References

#569330

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Before	After

How to set up and validate locally

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