google: move bigquery openlineage imports inside methods

[ahidalgob]

closes: #40043

In the latest google providers version (10.19.0), importing bigquery operators results in importing the openlineage package even when the openlineage provider is not installed.

This refactors the code so all imports to the openlineage package happen only inside get_openlineage_facets_on_* again.

#39614 moved the mixin to openlineage/utils.py and this brings it back to operators/bigquery.py to keep the top-level openlineage imports in utils.py, and even worse because the mixin is now bulkier thank before. Another option would be to move BigQueryJobRunFacet and BigQueryErrorRunFacet to another file, and move all openlineage top-level imports in utils.py inside methods. Open to suggestions.

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[VladaZakharova]

LGTM
Great job!

[JDarDagran]

Would be more feasible to keep the code in the separate module as it is and move OL imports from top-level to local imports?

[ahidalgob]

Would be more feasible to keep the code in the separate module as it is and move OL imports from top-level to local imports?

@JDarDagran The problem I had trying this was the definition of BigQueryJobRunFacet and BigQueryErrorRunFacet as they need the top level BaseFacet to inherit from. That's why my alternative was to create another file for facets, but doesn't looks clean either.

[JDarDagran]

How about having mixin only in, let's say, airflow/providers/google/cloud/openlineage/mixins.py, in which all imports are local?
I can attempt to fix it in another PR or leave it up to you in this one.

[ahidalgob]

sounds good to me, I will make the changes in this PR