[in_app_purchase] Return jwsRepresentation and jsonRepresentation for StoreKit2

[shaunpanjabi]

Fix for flutter/flutter#164433

Providing access to jwsRepresentation. property of the transaction. With the most recent update PurchaseVerificationData returns empty now. I think it could make sense to repurpose it and return the:

• jsonRepresentation for localVerificationData
• jwsRepresentation for serverVerificationData

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[maks-epowar]

Hi is there any update on adding this? This is more logical than returning empty verification data