Rewrite to use core's Default Content API

The format that core uses is identical to default content, we don't need to rewrite it.

It's still internal, but we can try running our tests against the core implementation.

Started with an initial test of replacing the importer completely. Tests are failing, one case seems to be that it doesn't like non-existing fields.

A more feasible approach is likely to wrap it inside since the API has changed, that should make the transition easier, and we could then look into deprecating our API.

rebased on top of latest 2.0.x and except changes in \Drupal\default_content\Importer::importContent() looks it no longer can import hal_json which means that's 3.x material

PS: Using PHP 8.4 showing lots of warnings so filed 📌 [PHP 8.4] Fix implicitly nullable type declarations Needs review