Allow re-use of decorated task groups

[Pocoder]

There is no way to change group_id of @task_group decorated function (TaskGroupDecorator) outside of its declaration.
I am following the same style that was proposed in solving the same problem for @task decorated functions #22941:

@task_group
def simple_tg():
   ...

@dag
def simple_dag():
    tg = simple_tg.override(group_id='other_id')()

Also this feature allows to change other kwargs of TaskGroup:

@dag
def simple_dag2():
    tg = simple_tg.override(default_args={
        'pool': 'mypool',
    })()

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[boring-cyborg]

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[potiuk]

@uranusjr - wdyt? Seems like it might work.

[uranusjr]

Makes sense

[josh-fell]

This will be great for reusing collections of tasks among repeatable patterns in DAGs. ❤️

[boring-cyborg]

Awesome work, congrats on your first merged pull request!