**Apache Airflow version**: 2.0.0

**What happened**:

Using endpoint `/dags/{dag_id}/dagRuns/{dag_run_id}/taskInstances/{task_id}/xcomEntries/{xcom_key}` I got Response Body but without `value` entry. Like:
```
{
  "dag_id": "string",
  "execution_date": "string",
  "key": "string",
  "task_id": "string",
  "timestamp": "string"
}
```
Instead of:
```
{
  "dag_id": "string",
  "execution_date": "string",
  "key": "string",
  "task_id": "string",
  "timestamp": "string",
  "value": "string"
}
```
The exact value by defined `key` exists.
tests/api_connexion/endpoints/test_xcom_endpoint.py
url = "/api/v1/dags/{dag_id}/dagRuns/{dag_run_id}/taskInstances/{task_id}/xcomEntries?{query_params}"
        url = url.format(
            dag_id=self.dag_id, dag_run_id=self.dag_run_id, task_id=self.task_id, query_params=query_params
        )
        dagrun = DR(
            dag_id=self.dag_id,
            run_id=self.dag_run_id,
            execution_date=self.execution_date_parsed,
            start_date=self.execution_date_parsed,
            run_type=DagRunType.MANUAL,
        )
        xcom_models = self._create_xcoms(10)
        session.add_all(xcom_models)
        session.add(dagrun)
        session.commit()
        response = self.client.get(url, environ_overrides={'REMOTE_USER': "test"})
        assert response.status_code == 200
        self.assertEqual(response.json["total_entries"], 10)
        conn_ids = [conn["key"] for conn in response.json["xcom_entries"] if conn]
        self.assertEqual(conn_ids, expected_xcom_ids)
    def _create_xcoms(self, count):