Add missing GraphQL fields

[samalisam-novu]

Several properties returned by the SEO cascade were not exposed in the GraphQL schema, making them inaccessible to headless frontends.

This PR adds the following fields to the SeoPro GraphQL type:

• json_ld -> returns a list of JSON-LD snippet strings (site defaults + section defaults or entry settings)
  • Maybe not necessary to seperate site defaults from the rest, but makes life easier in case you want it separately.
• robots
• robots_indexing
• is_default_site
• current_hreflang