Sync prompt API

Assuming that implementing websocket adds complexity to other systems that use ComfyUI API, having a sync HTTP endpoint that awaits the workflow to finish can be a solution.

To implement, this instructions should work:

- Add `POST /prompt_sync` endpoint. This endpoint waits the prompt_id to be executed, adds a callback function on a map.
- Add `POST /prompt_finish` endpoint. This endpoint triggers a callback function from a map according to prompt_id
- Add node `Finish workflow with image`. This node trigers ` /prompt_finish` with the image and prompt_id