Add App Store Server API v2, Server Notifications & CloudKit Push Notifications

[meszmate]

Summary

• App Store Server Notifications — Parse and verify V1/V2 webhook payloads with JWS signature verification against the Apple Root CA - G3 certificate chain. Includes DecodeTransactionInfo and DecodeRenewalInfo for signed JWS fields.
• App Store Server API v2 — Full HTTP client for all 12 Apple endpoints with JWT Bearer authentication (ES256). Covers transactions, subscriptions, order lookups, refund history, consumption info, and notification management. Includes ASAPIError type with all documented Apple error codes and Retry-After header support.
• CloudKit Push Notification Parsing — ParseCKPushNotification() function to parse incoming APNs payloads from CloudKit query and record zone subscriptions.
• README updated with documentation and usage examples for all new features