add first draft comfy api example

[kning]

Basically modal app version of the comfyUI websockets example.

This passes a comfy workflow json to a running comfyUI server, queues a job, and fetches the image outputs of that job via API.

Chatted with @thundergolfer on how this could be expanded into a more standalone example:

• Put a simple frontend (like this example) that allows a user to adjust the editable parts of a comfyui json workflow (e.g. text prompts).
• Use this code as a starting point web endpoint to handle those requests and create the workflow json, spin up a "headless" comfyui server, and return the output

Other thoughts:

• fal.ai allows you to pass urls of models
• how would people adapt this if their workflow included plugins?

[thundergolfer]

Superseded by #628