**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.
airflow/api_connexion/endpoints/xcom_endpoint.py
from airflow.models import DagRun as DR, XCom
from airflow.security import permissions
from airflow.utils.session import provide_session
@security.requires_access(
    [
        (permissions.ACTION_CAN_READ, permissions.RESOURCE_DAG),
        (permissions.ACTION_CAN_READ, permissions.RESOURCE_DAG_RUN),
        (permissions.ACTION_CAN_READ, permissions.RESOURCE_TASK_INSTANCE),
        (permissions.ACTION_CAN_READ, permissions.RESOURCE_XCOM),
    ]
)
@format_parameters({'limit': check_limit})
@provide_session
def get_xcom_entries(
    dag_id: str,
    dag_run_id: str,
    task_id: str,
    session: Session,
    limit: Optional[int],
    offset: Optional[int] = None,
) -> XComCollectionSchema:
    """Get all XCom values"""
    query = session.query(XCom)
    if dag_id == '~':
        appbuilder = current_app.appbuilder
        readable_dag_ids = appbuilder.sm.get_readable_dag_ids(g.user)
        query = query.filter(XCom.dag_id.in_(readable_dag_ids))
        query = query.join(DR, and_(XCom.dag_id == DR.dag_id, XCom.execution_date == DR.execution_date))
    else:
        query = query.filter(XCom.dag_id == dag_id)