Add a JSON Schema for what the JSON model should look like and validate against invalid JSON input

See: https://github.com/openfga/openfga/issues/1164

The below model is invalid because the intersection definition in the document#viewer relation is malformed. It’s missing a child: […] nested block.

{
  "schema_version": "1.1",
  "type_definitions": [
    {
        "type": "user"
    },
    {
        "type": "folder",
        "relations": {
            "viewer": {"this": {}},
            "parent": {"this": {}}
        },
        "metadata": {
            "relations": {
                "viewer": {
                    "directly_related_user_types": [{"type": "user"}]
                },
                "parent": {
                    "directly_related_user_types": [{"type": "folder"}]
                }
            }
        }
    },
    {
        "type": "document",
        "relations": {
            "parent": {"this": {}},
            "viewer": {
                "union": {
                    "child": [
                        {
                            "intersection": {
                                "tupleToUserset": {
                                    "tupleset": {
                                        "relation": "parent"
                                    },
                                    "computedUserset": {
                                        "relation": "viewer"
                                    }
                                }
                            }
                        },
                        {"this": {}}
                    ]
                }
            }
        },
        "metadata": {
            "relations": {
                "parent": {
                    "directly_related_user_types": [{"type": "folder", "relation": "parent"}]
                },
                "viewer": {
                    "directly_related_user_types": [{"type": "user"}]
                }
            }
        }
    }
  ]
}

The issue provides a malformed JSON model in its body and links to openfga/openfga#1164, but names no files, tests, or validation entry point. Start by locating the JSON model representation and its current parsing or validation path. Done means a JSON Schema exists and invalid input such as the supplied intersection example is rejected.