Push XCom if `BaseOperator.do_xcom_push` is True and if task returns a value

[kaxil]

Add support for pushing XCom values by just returning from a function.

def push_by_returning(**kwargs):
    """Pushes an XCom without a specific target, just by returning it"""
    return "hi from push_by_returning"

push2 = PythonOperator(
    task_id="push_by_returning",
    dag=dag,
    python_callable=push_by_returning,
)

Currently, we support explicit ti.xcom_push and pull with Task SDK