Wrap single NodesSources responses in a RootObject to limit serialization side-effects

[ambroisemaupate]

When we inject blocks or breadcrumbs into a NodesSources they might be serialized again and injected for any child NodesSources wasting precious TTFB and response size.

If we wrap single NodesSources responses in a RootObject, we will be able to add additional data outside of NodesSources:

{
  "@type": "SingleNodesSourcesRoot",
  "item": {
    "@id": "",
    "@type": "Page",
    "someNodeRef": [{
      "@id": "",
      "@type": "BlogPost"
    }],
  },
  "blocks": {},
  "breadcrumbs": []
}

instead of:

{
  "@id": "",
  "@type": "Page",
  "someNodeRef": [{
    "@id": "",
    "@type": "BlogPost",
    "blocks": {}, 
    "breadcrumbs": []
  }],
  …
  "blocks": {},
  "breadcrumbs": []
}