add more payment related shit

pkg/cart/mutation_payment_event.go

@@ -0,0 +1,21 @@
+package cart
+
+import (
+	"encoding/json"
+
+	messages "git.k6n.net/go-cart-actor/pkg/messages"
+)
+
+func PaymentEventHandler(grain *CartGrain, req *messages.PaymentEvent) error {
+
+	payment, found := grain.FindPayment(req.PaymentId)
+	if !found {
+		return ErrPaymentNotFound
+	}
+	metaBytes := req.Data.GetValue()
+	payment.Events = append(payment.Events, &PaymentEvent{
+		Name: req.Name,
+		Data: json.RawMessage(metaBytes),
+	})
+	return nil
+}