**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
'key': 'test-xcom-key-1',
                        'task_id': task_id_1,
                        'timestamp': "TIMESTAMP",
                    },
                    {
                        'dag_id': dag_id_1,
                        'execution_date': execution_date,
                        'key': 'test-xcom-key-2',
                        'task_id': task_id_1,
                        'timestamp': "TIMESTAMP",
                    },
                    {
                        'dag_id': dag_id_2,
                        'execution_date': execution_date,
                        'key': 'test-xcom-key-1',
                        'task_id': task_id_2,
                        'timestamp': "TIMESTAMP",
                    },
                    {
                        'dag_id': dag_id_2,
                        'execution_date': execution_date,
                        'key': 'test-xcom-key-2',
                        'task_id': task_id_2,
                        'timestamp': "TIMESTAMP",
                    },
                ],
                'total_entries': 4,
            },
        )
    def test_should_respond_200_with_tilde_and_granular_dag_access(self):