feat(epay): support type selectors and preserve callback type

src/model/request/order_request.go

@@ -18,6 +18,7 @@ type CreateTransactionRequest struct {
 	// empty or any other value is stored as "Gmpay" and uses GMPay JSON.
 	// It is optional for GMPay, but must be included in the signature when sent.
 	PaymentType string `json:"payment_type" form:"payment_type" example:"Epay"`
+	EpayType    string `json:"-" form:"-"`
 }
 
 func (r CreateTransactionRequest) Translates() map[string]string {