openlineage: Set slots to True for facets used in dag level

[JDarDagran]

Implements approach proposed in #40971.

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[kacpermuda]

Quick question, if the default behaviour changed and all Facets in v2 have the slots=True why not change all the Airflow facets to mimic that and not only the ones that go to DAG events and undergo pickling. Is that impacting the performance a lot? Is there any other point here that we are worried about? For the sake of simplicity and to avoid mistakes i think it would be good to keep it consistent. If not, let's at least leave a comment / docstring somewhere in the code about why sometimes it's False and sometimes it's True.

[JDarDagran]

Quick question, if the default behaviour changed and all Facets in v2 have the slots=True why not change all the Airflow facets to mimic that and not only the ones that go to DAG events and undergo pickling. Is that impacting the performance a lot? Is there any other point here that we are worried about? For the sake of simplicity and to avoid mistakes i think it would be good to keep it consistent. If not, let's at least leave a comment / docstring somewhere in the code about why sometimes it's False and sometimes it's True.

I agree, for the sake of simplicity it'd be better to have all facets set slots to True.