Add toString method in DefaultHttp2WindowUpdateFrame (#10763)

Motivation:
We should have the `toString` method in `DefaultHttp2WindowUpdateFrame` because it makes debugging a little easy.

Modification:
Added `toString` method.

Result:
`toString` method to help in debugging.


Co-authored-by: Norman Maurer <norman_maurer@apple.com>

codec-http2/src/main/java/io/netty/handler/codec/http2/DefaultHttp2WindowUpdateFrame.java

@@ -15,6 +15,7 @@
  */
 package io.netty.handler.codec.http2;
 
+import io.netty.util.internal.StringUtil;
 import io.netty.util.internal.UnstableApi;
 
 /**
@@ -44,4 +45,10 @@ public String name() {
     public int windowSizeIncrement() {
         return windowUpdateIncrement;
     }
+
+    @Override
+    public String toString() {
+        return StringUtil.simpleClassName(this) +
+                "(stream=" + stream() + ", windowUpdateIncrement=" + windowUpdateIncrement + ')';
+    }
 }