**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 with DAGs having cyclic dependencies set up by ExternalTaskMarker and
    ExternalTaskSensor.
    dag_0:   task_a_0 >> task_b_0
                  ^          |
                  |          |
    dag_1:        |          ---> task_a_1 >> task_b_1
                  |                              ^
                  |                              |
    dag_n:        |                              ---> task_a_n >> task_b_n
                  |                                                   |
                  -----------------------------------------------------
    """
    def _factory(depth: int) -> DagBag:
        dag_bag = DagBag(dag_folder=DEV_NULL, include_examples=False)
        dags = []
        with DAG("dag_0", start_date=DEFAULT_DATE, schedule_interval=None) as dag:
            dags.append(dag)
            task_a_0 = DummyOperator(task_id="task_a_0")
            task_b_0 = ExternalTaskMarker(
                task_id="task_b_0", external_dag_id="dag_1", external_task_id="task_a_1", recursion_depth=3
            )
            task_a_0 >> task_b_0
        for n in range(1, depth):
            with DAG(f"dag_{n}", start_date=DEFAULT_DATE, schedule_interval=None) as dag:
                dags.append(dag)
                task_a = ExternalTaskSensor(
                    task_id=f"task_a_{n}",