Support for Additional Currency in IPaymentItem

The interface IPaymentItem now includes the property CurrencyInfo, which allows storing data about an additional currency for the payment. To support the use of additional currencies in payments, the methods AddPaymentItem, AddExternalPaymentItem, and AddExternalFiscalizedPaymentItem have been enhanced to allow specifying an amount in an additional currency, as well as indicating this currency through the interface IAdditionalCurrency. Additionally, a method for retrieving a list of currencies with their current exchange rates, GetCurrencyRates, has been added, as well as a method for retrieving a list of currencies with rates for a specific order, GetCurrencyRatesForOrder.