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.

config.go

@@ -0,0 +1,23 @@
+package mrktplaats
+
+import "context"
+
+// ConfigService handles A/B tests and feature flags.
+type ConfigService struct {
+	t *transport
+}
+
+// LabsConfig contains A/B test assignments and feature flags.
+type LabsConfig struct {
+	ABSwitches     map[string]string `json:"abSwitches"`
+	FeatureSwitches map[string]bool  `json:"featureSwitches"`
+}
+
+// Labs returns the current A/B test and feature flag configuration.
+func (s *ConfigService) Labs(ctx context.Context) (*LabsConfig, error) {
+	var resp LabsConfig
+	if err := s.t.get(ctx, "/app/v1/labs/config", nil, &resp); err != nil {
+		return nil, err
+	}
+	return &resp, nil
+}