Add Amazon AppStore IAP support

[Copilot]

Changes proposed in this pull request:

• Add Amazon AppStore platform adapter with full IAP SDK 3.0.8 integration
• Implement native Android bridge (AmazonPurchasePlugin.java) supporting product queries, purchases, receipt updates, and fulfillment
• Add TypeScript adapter, bridge, and type definitions following existing platform patterns
• Update README features matrix to include Amazon AppStore support

Platform Integration

• AMAZON_APPSTORE platform constant added to enum
• Adapter registered in core adapters.ts initialization flow
• Validator types defined for Amazon receipt verification
• Plugin.xml configured with amazon-appstore-sdk:3.0.8 dependency

Supported Operations

• Product loading via PurchasingService.getProductData()
• Purchase flow via PurchasingService.purchase()
• Receipt restoration via PurchasingService.getPurchaseUpdates()
• Consumable fulfillment via PurchasingService.notifyFulfillment()

Usage

await store.initialize([
  Platform.GOOGLE_PLAY,
  Platform.AMAZON_APPSTORE, // Now supported
]);

Note: Amazon IAP SDK provides only formatted price strings. The adapter handles this with optional priceMicros and currency fields.

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To test this pull request with cordova:

# 1: Uninstall the plugin (if already installed)
cordova plugin rm cordova-plugin-purchase

# 2: Install from github
cordova plugin add "https://github.com/j3k0/cordova-plugin-purchase.git#v13.12-amazon"

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