Add GraphQL mutation to associate a upstream to registry

What does this MR do and why?

Adds a new GraphQL mutation that allows for a container upstream to be associated with a registry.

mutation {
  containerVirtuaRegistryUpstreamCreate(input:{registryId:"gid://gitlab/VirtualRegistries::Container::Registry/9", upstreamId:"gid://gitlab/VirtualRegistries::Container::Upstream/7"}) {
    errors
    registryUpstream {
      id
      position
      registry {
        id
      }
    }
  }
}

References

#582490 (closed)

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