v5.5.1

• Replaced untyped PurchaseOptions (standard/subscription as object) with fully typed interfaces:
  StandardPurchaseOption, SubscriptionPurchaseOption, SubscriptionPlan
• Reused SubscriptionBillingSchedule / SubscriptionOrderSchedule from types/subscription instead of
  duplicating
• Fixed OptionValue.subscription_interval type from number to string enum
• Added type field to Product
• Added camel-case variants for all new types