**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
def setUp(self):
        super().setUp()
        self.dag_id = 'test-dag-id'
        self.task_id = 'test-task-id'
        self.execution_date = '2005-04-02T00:00:00+00:00'
        self.execution_date_parsed = parse_execution_date(self.execution_date)
        self.dag_run_id = DR.generate_run_id(DagRunType.MANUAL, self.execution_date_parsed)
    @parameterized.expand(
        [
            (
                "limit=1",
                ["TEST_XCOM_KEY1"],
            ),
            (
                "limit=2",
                ["TEST_XCOM_KEY1", "TEST_XCOM_KEY10"],
            ),
            (
                "offset=5",
                [
                    "TEST_XCOM_KEY5",
                    "TEST_XCOM_KEY6",
                    "TEST_XCOM_KEY7",
                    "TEST_XCOM_KEY8",
                    "TEST_XCOM_KEY9",
                ],
            ),
            (