create the interface the PSP modules have to implement

[MGA-dotSource]

This interface is intended to be implemented by the payment service provider specific implemented modules and is the one that is autodiscovered by #4

It provides the following methods:

• getAvailablePaymentMethods - returns a list of PaymentMethodInfo objects, consumes a filter data wrapper object that allows passing available data for future filter purposes -> right now these are ignored and no filtering should take place
• provideCreatePaymentHandler - returns an executable function object that is responsible for creating a payment object at the CTP, consumes a methodID parameter that allows returning a specific handler
• provideCreatePaymentTransactionHandler - return an executable function object that is responsible for creating a payment transaction for an existing payment object
• provideGetPaymentStatusHandler - return an executable function that returns a payment status object for a passed payment reference

[MGA-dotSource]

basic interface is created, detailed return values will be discussed within separate tickets