Description

Enable the ability for a task in a DAG to wait upon the successful completion of an entire TaskGroup.

Use case / motivation

TaskGroups provide a great mechanism for authoring DAGs, however there are situations where it might be necessary for a task in an external DAG to wait upon the the completion of the TaskGroup as a whole.

At the moment this is only possible with one of the following workarounds:

1. Add an external task sensor for each task in the group.
2. Add a Dummy task after the TaskGroup which the external task sensor waits on.

I would envisage either adapting ExternalTaskSensor to also work with TaskGroups or creating a new ExternalTaskGroupSensor.

Are you willing to submit a PR?

Time permitting, yes!

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