feat: add aag callback example

Author: xaviergoh-bytedance

volcengine-java-sdk-ark-runtime/test/java/com/volcengine/ark/runtime/ContentGenerationTaskExample.java

@@ -50,6 +50,7 @@ public static void main(String[] args) {
         CreateContentGenerationTaskRequest createRequest = CreateContentGenerationTaskRequest.builder()
                 .model(model)
                 .content(contents)
+                // .callbackUrl("YOUR CALLBACK URL")
                 .build();
 
         // send create request