Add support for Kubernetes Gateway API RequestHeaderModifier filter

Co-authored-by: Baptiste Mayelle <baptiste.mayelle@traefik.io>

pkg/middlewares/headermodifier/request_header_modifier.go

@@ -0,0 +1,56 @@
+package headermodifier
+
+import (
+	"context"
+	"net/http"
+
+	"github.com/traefik/traefik/v3/pkg/config/dynamic"
+	"github.com/traefik/traefik/v3/pkg/middlewares"
+	"go.opentelemetry.io/otel/trace"
+)
+
+const typeName = "RequestHeaderModifier"
+
+// requestHeaderModifier is a middleware used to modify the headers of an HTTP request.
+type requestHeaderModifier struct {
+	next http.Handler
+	name string
+
+	set    map[string]string
+	add    map[string]string
+	remove []string
+}
+
+// NewRequestHeaderModifier creates a new request header modifier middleware.
+func NewRequestHeaderModifier(ctx context.Context, next http.Handler, config dynamic.RequestHeaderModifier, name string) (http.Handler, error) {
+	logger := middlewares.GetLogger(ctx, name, typeName)
+	logger.Debug().Msg("Creating middleware")
+
+	return &requestHeaderModifier{
+		next:   next,
+		name:   name,
+		set:    config.Set,
+		add:    config.Add,
+		remove: config.Remove,
+	}, nil
+}
+
+func (r *requestHeaderModifier) GetTracingInformation() (string, string, trace.SpanKind) {
+	return r.name, typeName, trace.SpanKindUnspecified
+}
+
+func (r *requestHeaderModifier) ServeHTTP(rw http.ResponseWriter, req *http.Request) {
+	for headerName, headerValue := range r.set {
+		req.Header.Set(headerName, headerValue)
+	}
+
+	for headerName, headerValue := range r.add {
+		req.Header.Add(headerName, headerValue)
+	}
+
+	for _, headerName := range r.remove {
+		req.Header.Del(headerName)
+	}
+
+	r.next.ServeHTTP(rw, req)
+}