Support Menu Item Extra fields for Menu items

We did this, but made it hardcoded only for the fields we needed, you can take a look in our module for inspiration.

Thanks @CedricL, I'll definitely use it as inspiration and hopefully as a basis for contributing.

Howdy, MR is in, please test and let me know.
Because we don't "out of the box" support a union of MenuItem types, I've added it to an `extras` field.

Then I messed about with some of the form UX to make it less.. crap to work with.

Enable the MenuLinkContent entity fields on Menu in the GraphQL Compose schema form.

Query on the MenuItem.extras union.

{
  menu(name: MAIN) {
    items {
      extras {
        ... on MenuLinkContentMain {
          kitties
        }
      }
    }
  }
}

Also added on MenuItem.langcode → Language

I tested on the D10.8 site and it returned the following error: For error #0: Error: Call to protected method Drupal\menu_link_content\Plugin\Menu\MenuLinkContent::getEntity() from scope Drupal\graphql_compose_menus\Plugin\GraphQL\DataProducer\MenuTreeLinkEntity in Drupal\graphql_compose_menus\Plugin\GraphQL\DataProducer\MenuTreeLinkEntity->resolve() (line 45 of /app/web/modules/contrib/graphql_compose/modules/graphql_compose_menus/src/Plugin/GraphQL/DataProducer/MenuTreeLinkEntity.php)

Seems like the `getEntity()` method is public until D10.2 → and many sites are not ready to move to D10.2

Ok,
So thats updated now to use a buffer again.

Wondering if the semantics are now correct.
is extras ok?
should it be fields

Or do we completely mess with the menu system and clean it up to make more sense if this is an actual entity now.

Keen for feedback, as i dont feel like going...

menu(name: MAIN) {
  items {
    extras {
      thing
    }
  }
}

Really means anything to anyone. It feels like a total after thought. Theres gotta be an "if i didn't have to support any legacy stuff, this is the ideal structure" moment, somewhere?

Have a plan
Keen on feedback

Without trying to over think this more, I feel like ‘fields’ makes the most sense here.

Will write up some tests tonight and get this in.

Beta 2 tagged, thanks all