**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
else:
                raise e
        dag_id = TEST_DAG_ID
        dag = DAG(dag_id, default_args=self.args, schedule_interval=timedelta(minutes=1))
        task_without_failure = ExternalTaskSensor(
            task_id='task_without_failure',
            external_dag_id=dag_external_id,
            external_task_id='task_external_without_failure',
            execution_date_fn=lambda dt: [dt + timedelta(seconds=i) for i in range(2)],
            allowed_states=['success'],
            retries=0,
            timeout=1,
            poke_interval=1,
            dag=dag,
        )
        task_with_failure = ExternalTaskSensor(
            task_id='task_with_failure',
            external_dag_id=dag_external_id,
            external_task_id='task_external_with_failure',
            execution_date_fn=lambda dt: [dt + timedelta(seconds=i) for i in range(2)],
            allowed_states=['success'],
            retries=0,
            timeout=1,
            poke_interval=1,
            dag=dag,
        )
        task_without_failure.run(start_date=DEFAULT_DATE, end_date=DEFAULT_DATE, ignore_ti_state=True)
        with pytest.raises(AirflowSensorTimeout):
            task_with_failure.run(start_date=DEFAULT_DATE, end_date=DEFAULT_DATE, ignore_ti_state=True)
    def test_external_task_sensor_delta(self):