Request to add purchaseOptionId to OneTimePurchaseOfferDetails

[simmotde]

Android BillingClient has a function getPurchaseOptionId(). Can this be added as purchaseOptionId to OneTimePurchaseOfferDetails?

This will help in determining which purchase option to show to user.

[hyochan]

PR created: #78

Implemented the following fields from Google Play Billing Library:

BillingClient 7.0+:

• purchaseOptionId on ProductAndroidOneTimePurchaseOfferDetail
• purchaseOptionIdAndroid on DiscountOffer (standardized type)
• InstallmentPlanDetails type for subscription installment plans
• installmentPlanDetails on ProductSubscriptionAndroidOfferDetails
• installmentPlanDetailsAndroid on SubscriptionOffer (standardized type)

BillingClient 5.0+:

• PendingPurchaseUpdate type for subscription upgrades/downgrades
• pendingPurchaseUpdateAndroid on PurchaseAndroid

All fields are extracted via runCatching for backward compatibility with older Billing Library versions.

Note: OpenIAP uses BillingClient 8.3.0+, which includes all features from 5.0+ and 7.0+. The version numbers indicate when each feature was introduced, not the minimum required version.