Add send send_sync clear execution_finished message with seconds

Would be possible to make clear in the websocket that the execution finished by adding it after this lines:
https://github.com/Comfy-Org/ComfyUI/blob/2f4c4e983c63dc60ae781bcca01e0e17f4f404d6/main.py#L358-L362

Example:

```
if execution_time > 600:
execution_time = time.strftime("%H:%M:%S", time.gmtime(execution_time))
logging.info(f"Prompt executed in {execution_time}", extra={'color': 'green'})
else:
logging.info("Prompt executed in {:.2f} seconds".format(execution_time), extra={'color': 'green'})
server_instance.send_sync("execution_finished", {"prompt_id": prompt_id, "execution_time": execution_time})
```

In some cases I'm just getting 'status' messages without prompt_id in the data, and that's the last message of the render..
Adding this extra message will make things super clear, we could change the execution_finished to something else if this conflicts with other messages.

Thanks!