Use only single execution date in ExternalTaskSensor

**Description**

In general we encourage operators to be designed to perform an atomic action. However, `ExternalTaskSensor` seems to be not alway atomic. Users are able to pass `execution_date_fn` which can return multiple dates. If that happens the operator will check state not of single DAG/task but of multiple instances. Even if that happens users will get 1/0 response - task/dag failed or not.

In my opinion the proper way to handle the multiple dates case would be to have multiple instances of the sensor, each checking for a task/dag for a given date.

**Use case / motivation**

Improve atomicity of `ExternalTaskSensor`.

**Related Issues**

_Originally posted by @turbaszek in https://github.com/apache/airflow/pull/12574#discussion_r528967901_