Add optional webhook after job finished

[pawelkania]

Add optional webhook after job finished, allow to send prompt_id with api request

api params to send:

{
   "prompt": "json - comfyui workflow exported as api format",
   "prompt_id": "uuidv4", # optional - you can use it to reference your db record for job - webhook will contain this uuid
   "webhook_url": "url to send results from comfyui after job is complete",
}

Comfyui will rspond to webhook url with params:

{
  "prompt_id": "uuidv4 you provided during job creation (or autogenerated in comfyui during job creation)",
  "execution_time": 32.7151094869987, # (in seconds)
  "status": "success/error",
  "result": {"outputs": {}} # same format as in history endpoint
}