Move HTML rendering of fields to a sub-resource

[bdunogier]

Since simple field types have been changed to return a scalar, they can't be rendered to HTML anymore. To resolve this, HTML rendering of fields can be moved to its own field:

{
  content {
    blogPosts {
      _renderedFields {
        title(template: "foo.html.twig")
        authors
      }
    }
  }
}