Support NativeLB option in GatewayAPI provider

Co-authored-by: Kevin Pollet <pollet.kevin@gmail.com>

pkg/provider/kubernetes/gateway/annotations.go

@@ -0,0 +1,54 @@
+package gateway
+
+import (
+	"fmt"
+	"strings"
+
+	"github.com/traefik/traefik/v3/pkg/config/label"
+)
+
+const annotationsPrefix = "traefik.io/"
+
+// ServiceConfig is the service's root configuration from annotations.
+type ServiceConfig struct {
+	Service Service `json:"service"`
+}
+
+// Service is the service's configuration from annotations.
+type Service struct {
+	NativeLB bool `json:"nativeLB"`
+}
+
+func parseServiceAnnotations(annotations map[string]string) (ServiceConfig, error) {
+	var svcConf ServiceConfig
+
+	labels := convertAnnotations(annotations)
+	if len(labels) == 0 {
+		return svcConf, nil
+	}
+
+	if err := label.Decode(labels, &svcConf, "traefik.service."); err != nil {
+		return svcConf, fmt.Errorf("decoding labels: %w", err)
+	}
+
+	return svcConf, nil
+}
+
+func convertAnnotations(annotations map[string]string) map[string]string {
+	if len(annotations) == 0 {
+		return nil
+	}
+
+	result := make(map[string]string)
+
+	for key, value := range annotations {
+		if !strings.HasPrefix(key, annotationsPrefix) {
+			continue
+		}
+
+		newKey := strings.ReplaceAll(key, "io/", "")
+		result[newKey] = value
+	}
+
+	return result
+}