Initial Marktplaats SDK scaffold

Clone the twdehands SDK into mrktplaats and retarget naming and defaults for app.marktplaats.nl while keeping the same request/response bodies and endpoint behavior.

payments.go

@@ -0,0 +1,23 @@
+package mrktplaats
+
+import "context"
+
+// PaymentService handles payment cart operations.
+type PaymentService struct {
+	t *transport
+}
+
+// PaymentCartCountResponse contains the number of items in the payment cart.
+type PaymentCartCountResponse struct {
+	UserID        string `json:"userId"`
+	NumberOfItems int    `json:"numberOfItems"`
+}
+
+// CartItemCount returns the number of items in the payment cart.
+func (s *PaymentService) CartItemCount(ctx context.Context) (*PaymentCartCountResponse, error) {
+	var resp PaymentCartCountResponse
+	if err := s.t.get(ctx, "/app/payments/v3/payment-cart/number-of-items", nil, &resp); err != nil {
+		return nil, err
+	}
+	return &resp, nil
+}