feat: ark content generation sdk image roles example

Author: fabianlimxuhuai

volcenginesdkexamples/volcenginesdkarkruntime/content_generation_tasks.py

@@ -21,6 +21,7 @@
                 "image_url": {
                     "url": "${YOUR_IMAGE_URL}"
                 },
+                # "role": "first_frame"
             }
         ]
     )