update RussiaPaymentOrder payload

[AlexandreZaytsev]

Raffael Hi
please take a look I suggest changing the RussiaPaymentOrder code to three constructors (three data structures)

RussiaPaymentOrder(
	MandatoryFields mandatoryFields,
	CharacterSets characterSet = CharacterSets.utf_8)
RussiaPaymentOrder(
	MandatoryFields mandatoryFields, 
	OptionalFields optionalFields = null, 
	CharacterSets characterSet = CharacterSets.utf_8)
RussiaPaymentOrder(
	MandatoryFields mandatoryFields,
        OptionalFields optionalFields = null,
        OptionalExtFields optionalExtFields = null,
        CharacterSets characterSet = CharacterSets.utf_8)

the initializer is not quite convenient here - it is the constructors that are needed

they can be arranged separately in the form of classes (like the iban class in SwissQrCode : Payload)

and add non-empty field validation when generating a string - to pass the <300 character condition (like here (ContactData : Payload)
...
unfortunately, I do not yet have enough experience - how it is beautiful to design
ps
by the way - function ValidateInput (...- it turned out beautifully
pps
I made a fork of the pay load generator.cs file - I hope I didn't spoil anything in your project