Add ReceiptItem to VerifyPurchaseResult, VerifySubscriptionResult

[bizz84]

This PR introduces a new strong-typed ReceiptItem struct:

public struct ReceiptItem {
    // The product identifier of the item that was purchased. This value corresponds to the productIdentifier property of the SKPayment object stored in the transaction’s payment property.
    public let productId: String
    // The number of items purchased. This value corresponds to the quantity property of the SKPayment object stored in the transaction’s payment property.
    public let quantity: Int
    // The transaction identifier of the item that was purchased. This value corresponds to the transaction’s transactionIdentifier property.
    public let transactionId: String
    // For a transaction that restores a previous transaction, the transaction identifier of the original transaction. Otherwise, identical to the transaction identifier. This value corresponds to the original transaction’s transactionIdentifier property. All receipts in a chain of renewals for an auto-renewable subscription have the same value for this field.
    public let originalTransactionId: String
    // The date and time that the item was purchased. This value corresponds to the transaction’s transactionDate property.
    public let purchaseDate: Date
    // For a transaction that restores a previous transaction, the date of the original transaction. This value corresponds to the original transaction’s transactionDate property. In an auto-renewable subscription receipt, this indicates the beginning of the subscription period, even if the subscription has been renewed.
    public let originalPurchaseDate: Date
    // The primary key for identifying subscription purchases.
    public let webOrderLineItemId: String
    // The expiration date for the subscription, expressed as the number of milliseconds since January 1, 1970, 00:00:00 GMT. This key is only present for auto-renewable subscription receipts.
    public let subscriptionExpirationDate: Date?
    // For a transaction that was canceled by Apple customer support, the time and date of the cancellation. Treat a canceled receipt the same as if no purchase had ever been made.
    public let cancellationDate: Date?

    public let isTrialPeriod: Bool
}

This is parsed from the receipt and returned as part of the verifySubscription and verifyPurchase methods:

// Result for Consumable and NonConsumable
public enum VerifyPurchaseResult {
    case purchased(item: ReceiptItem)
    case notPurchased
}

// Verify subscription result
public enum VerifySubscriptionResult {
    case purchased(expiryDate: Date, items: [ReceiptItem])
    case expired(expiryDate: Date, items: [ReceiptItem])
    case notPurchased
}

Note that when one or more subscriptions are found for a given product id, they are returned as a ReceiptItem array ordered by expiryDate, with the first one being the newest.

This is useful to get all the valid date ranges for a given subscription.

This PR is a superset of #192 and #190.