**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
with self.dag:
            sensor = ExternalTaskSensor(
                task_id='templated_task', external_dag_id='dag_{{ ds }}', external_task_id='task_{{ ds }}'
            )
        instance = TaskInstance(sensor, DEFAULT_DATE)
        instance.render_templates()
        assert sensor.external_dag_id == f"dag_{DEFAULT_DATE.date()}"
        assert sensor.external_task_id == f"task_{DEFAULT_DATE.date()}"
    def test_external_task_sensor_fn_multiple_execution_dates(self):
        bash_command_code = """
{% set s=execution_date.time().second %}
echo "second is {{ s }}"
if [[ $(( {{ s }} % 60 )) == 1 ]]
    then
        exit 1
fi
exit 0
"""
        dag_external_id = TEST_DAG_ID + '_external'
        dag_external = DAG(dag_external_id, default_args=self.args, schedule_interval=timedelta(seconds=1))