**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
"""
    Test clearing a dag that has multiple ExternalTaskMarker.
    sqlite3 parser stack size is 100 lexical items by default so this puts a hard limit on
    the level of nesting in the sql. This test is intentionally skipped in sqlite.
    """
    agg_dag = dag_bag_multiple.get_dag("agg_dag")
    for delta in range(len(agg_dag.tasks)):
        execution_date = DEFAULT_DATE + timedelta(days=delta)
        run_tasks(dag_bag_multiple, execution_date=execution_date)
    
    
    
    assert agg_dag.clear(start_date=execution_date, end_date=execution_date, dag_bag=dag_bag_multiple) == 51
@pytest.fixture
def dag_bag_head_tail():