**Description**
In its current shape the ExternalTaskSensor accepts either a single task_id or None to poll for the completion of a dag run. We have a use case where a dependent dag should poll for only certain list of tasks in the upstream dag. One option is to add N ExternalTaskSensor nodes if there are N nodes to be dependent on but those will be too many Sensor Nodes in the dag and can be avoided if the ExternalTaskSensor can accept a list of task_ids to poll for. 


**Use case / motivation**

We have a upstream dag that updates a list of hive tables that is further used by a lot of downstream dags. This dag updates 100s of hive tables but some of the downstream dag depends only upon 10-20 of these tables, There are multiple dags which depends upon varied list of hive tables from the upstream dag. 


**Are you willing to submit a PR?**

Yes, we are willing to submit a PR for this

**Related Issues**

Not that I am aware of . I did a search on the issue list. 

tests/sensors/test_external_task_sensor.py
parent_dag_0  task_0  task_0
                     |
                     |
                     v
     child_dag_1   task_1  task_1
    """
    dag_bag = DagBag(dag_folder=DEV_NULL, include_examples=False)
    day_1 = DEFAULT_DATE
    dag_0 = DAG("parent_dag_0", start_date=day_1, schedule_interval=None)
    task_0 = ExternalTaskMarker(
        task_id="task_0",
        external_dag_id="child_dag_1",
        external_task_id="task_1",
        execution_date=day_1.isoformat(),
        recursion_depth=3,
        dag=dag_0,
    )
    dag_1 = DAG("child_dag_1", start_date=day_1, schedule_interval=None)
    _ = ExternalTaskSensor(
        task_id="task_1",
        external_dag_id=dag_0.dag_id,
        external_task_id=task_0.task_id,
        execution_date_fn=lambda execution_date: day_1 if execution_date == day_1 else [],
        mode='reschedule',
        dag=dag_1,
    )
    for dag in [dag_0, dag_1]:
        dag_bag.bag_dag(dag=dag, root_dag=dag)
    return dag_bag
def run_tasks(dag_bag, execution_date=DEFAULT_DATE):