Fix DataFusion example type annotations

[Taragolis]

I've got this error/warning during breeze static-checks mypy-providers --all-files during non related to this provider changes.

tests/system/providers/google/cloud/datafusion/example_datafusion.py:322: note: "__call__" is considered instance variable, to make it class variable use ClassVar[...]
tests/system/providers/google/cloud/datafusion/example_datafusion.py:322: note: Error code "misc" not covered by "type: ignore" comment

I guess this happen due to wrong type annotation for Task Flow operator

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