Refactor imports from 'airflow-core' out of 'shared/stats'

[xBis7]

This patch refactors the Stats.initialize() method so that it won't import any packages from airflow-core.

Instead of that method making calculations and then initializing the correct factory object, it will accept the factory as a parameter.

The initialization logic will be moved under a utils file in airflow-core and a duplicate file in task-sdk. This is the best approach forward based on offline discussions.

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[xBis7]

@amoghrajesh @potiuk Can you take a look?