**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
"""
    Create a DagBag containing two DAGs, linked by multiple ExternalTaskMarker.
    """
    dag_bag = DagBag(dag_folder=DEV_NULL, include_examples=False)
    daily_dag = DAG("daily_dag", start_date=DEFAULT_DATE, schedule_interval="@daily")
    agg_dag = DAG("agg_dag", start_date=DEFAULT_DATE, schedule_interval="@daily")
    dag_bag.bag_dag(dag=daily_dag, root_dag=daily_dag)
    dag_bag.bag_dag(dag=agg_dag, root_dag=agg_dag)
    daily_task = DummyOperator(task_id="daily_tas", dag=daily_dag)
    start = DummyOperator(task_id="start", dag=agg_dag)
    for i in range(25):
        task = ExternalTaskMarker(
            task_id=f"{daily_task.task_id}_{i}",
            external_dag_id=daily_dag.dag_id,
            external_task_id=daily_task.task_id,
            execution_date="{{ macros.ds_add(ds, -1 * %s) }}" % i,
            dag=agg_dag,
        )
        start >> task
    yield dag_bag
@pytest.mark.quarantined
@pytest.mark.backend("postgres", "mysql")
def test_clear_multiple_external_task_marker(dag_bag_multiple):