[huawei_iap] Support for verifying signature about hms-flutter-plugin

[serdar-can]

Hi @deakjahn, there are three classes that return data signatures.

• After purchasing an item, Huawei IAP Flutter Plugin will parse the JSON response and creates a PurchaseResultInfo class which includes InAppPurchaseData and corresponding inAppDataSignature as a String.
• After consuming an item, Huawei IAP Flutter Plugin will parse the JSON response and creates a ConsumeOwnedPurchaseResult class which includes ConsumePurchaseData and corresponding dataSignature as a String.
• After requesting owned purchases, Huawei IAP Flutter Plugin will also parse the JSON response and creates an OwnedPurchasesResult which includes inAppPurchaseDataList as mentioned, and corresponding inAppSignature field in order to verify the purchases.

Huawei IAP Flutter Plugin parses the entire JSON response without any loss, hence you may use the mentioned fields. In later releases, we will also add a field that returns the original JSON response to fulfill your request.

[deakjahn]

Yes, sure, there's no problem with the signatures themselves, just that the original JSON itself is not yet available. Thanks for the inlcusion in the future.