**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
"""
    Run all tasks in the DAGs in the given dag_bag. Return the TaskInstance objects as a dict
    keyed by task_id.
    """
    tis = {}
    for dag in dag_bag.dags.values():
        for task in dag.tasks:
            ti = TaskInstance(task=task, execution_date=execution_date)
            tis[task.task_id] = ti
            ti.run()
            assert_ti_state_equal(ti, State.SUCCESS)
    return tis
def assert_ti_state_equal(task_instance, state):