**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
self.test_time_sensor()
        op = ExternalTaskSensor(
            task_id='test_external_task_sensor_check_delta',
            external_dag_id=TEST_DAG_ID,
            external_task_id=TEST_TASK_ID,
            execution_delta=timedelta(0),
            allowed_states=['success'],
            dag=self.dag,
        )
        op.run(start_date=DEFAULT_DATE, end_date=DEFAULT_DATE, ignore_ti_state=True)
    def test_external_task_sensor_fn(self):
        self.test_time_sensor()
        
        op1 = ExternalTaskSensor(
            task_id='test_external_task_sensor_check_delta_1',
            external_dag_id=TEST_DAG_ID,
            external_task_id=TEST_TASK_ID,
            execution_date_fn=lambda dt: dt + timedelta(0),
            allowed_states=['success'],
            dag=self.dag,
        )