Support Schema.org metatag

Took a quick look today.
Things to note:

• It's not really a metatag, it's a script tag with a JSON value, built using metatags
• It's only injected on page views via schema_metatag_page_attachments_alter
• The schema results can have breadcrumbs via current_route_match, which will need to resolve in a subrequest, which in turn needs the URL of the entity and fires off extra queries.

To execute it in a subrequest, it'll need the SubrequestBuffer which is a part of graphql_compose_routes

So we have a few choices:

1. Do support breadcrumbs and add the schema LD data via the route() InternalEntity
2. Do support breadcrumbs via entity canonical URL and add to every entity
3. Don't support breadcrumbs on the schema

And just spitballin' some ideas here:

1. Putting scripts on the RouteInternal feels a bit weird and detached.

2. I'd prefer to keep the ability to execute a SubrequestBuffer low, it's not an excellent way of doing things. So adding it to every entities metatag prop might allow attacking the schema.

3. Doable now if we add it to the metatag as something like MetaTagScript and I massage some data around.

So I'm leaning towards option 3, and stripping out any breadcrumbs from the entity.
Option 2 is bad juju
Option 1 is a backup plan to be explored.

Thoughts?

@almunnings yes, I will explore option 3 to expose it as MetaTagScript

Hows this...

  node(id: "...") {
    ... on MetaTagInterface {
      metatag {
        ... on MetaTagScript {
          tag
          content
          attributes {
            type
          }
        }
      }
    }
  }

<tag {...attributes}>content</tag>

<script type="application/ld+json">...</script>

I've not added any special work around breadcrumbs. I don't really want to enable a subrequest per entity. I don't think thats especially sane. Unsure what to do with that.

Merged into dev. Prepping for beta2.

Thanks!