Export StoreScope from airflow.sdk as public interface

[amoghrajesh]

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Custom state backends have to annotate all eight BaseStoreBackend methods with the TaskScope | AssetScope union, but the SDK only exported the two members, not the union itself. The single public name was missing, so the only way to get it was the private _shared path, which the object storage backend was already depending on. airflow.state already re-exported it on the core side; this brings the SDK in line.

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