**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 one DAG, with task "head" depending on task "tail" of the
    previous execution_date.
    20200501     20200502                 20200510
    +------+     +------+                 +------+
    | head |    -->head |    -->         -->head |
    |  |   |   / |  |   |   /           / |  |   |
    |  v   |  /  |  v   |  /           /  |  v   |
    | body | /   | body | /     ...   /   | body |
    |  |   |/    |  |   |/           /    |  |   |
    |  v   /     |  v   /           /     |  v   |
    | tail/|     | tail/|          /      | tail |
    +------+     +------+                 +------+
    """
    dag_bag = DagBag(dag_folder=DEV_NULL, include_examples=False)
    with DAG("head_tail", start_date=DEFAULT_DATE, schedule_interval="@daily") as dag:
        head = ExternalTaskSensor(