Add getter for Http3ErrorCode.code (#297)

Motivation:
Sometimes its useful to be able to also get the code as specified in the RFC.

Modifications:
Add getter.

Result:
Be able to also inspect the "raw code"

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

src/main/java/io/netty/incubator/codec/http3/Http3ErrorCode.java

@@ -126,4 +126,8 @@ public enum Http3ErrorCode {
     Http3ErrorCode(int code) {
         this.code = code;
     }
+
+    public int code() {
+        return code;
+    }
 }