Iterate the CheckoutRequest type

[nibhar]

The CheckoutRequest has some open questions:

• [ ] change shopLogoUrl to i.e. appLogoUrl
• [ ] are protocolSpecifics needed or can this be flattened into the paymentOption.
• [ ] amount or value for the to-be-paid amount in the smallest unit.
• [ ] move extraData from the base level of the request into the NimiqPaymentOptions

Also potentially the typing of the ParsedCheckoutRequest could be improved regarding versions 1 and 2.

To be done after a consolidated branch for the MultiCurrencyCheckout is available.