The target_type property is incorrectly of type DataReferenceTargetDefinition

PPMNMI as per the last comment.

I have a custom entity with a DER field that allows either users or nodes to be referenced. Here's an example jsonapi representation of that field:

"subject_entity": {
  "data": {
    "type": "user--user",
    "id": "02873322-eb7c-4984-bf32-7cb7e8ec1b68",
    "meta": {
      "target_type": "user",
      "drupal_internal__target_id": 1237
    }
  },
  "links": {
    "self": {
      "href": "https://my-site.com/jsonapi/my_entity/my_entity/cfbb318c-dc03-4061-b0a2-b123ac904a5c/relationships/subject_entity?resourceVersion=id%3A387"
    }
  }
}

Because this field can reference either users or nodes, I want to be able to filter by the target_type:

jsonapi/my_entity/my_entity?filter[subject_entity.meta.drupal_internal__target_id]=1237&filter[subject_entity.meta.target_type]=user

This fails:

"errors": [
  {
    "title": "Bad Request",
    "status": "400",
    "detail": "Invalid nested filtering. The field `target_type`, given in the path `subject_entity.meta.target_type`, does not exist.",

Worse, if I try to include that object in my query like this:

/jsonapi/my_entity/my_entity?include=subject_entity

I get this error:

"errors": [
        {
            "title": "Bad Request",
            "status": "400",
            "detail": "`subject_entity` is not a valid relationship field name. Possible values: revision_user, subject_entity, related_entity.",

I've applied the patch, and it fixes the issue of not being able to filter by target_type. It does not fix the inability to include the referenced data.