Feature request: node level metadata

As of now, third-party tools/frontends rely on workarounds to identify inputs to display on the user interface. For example:

- [StableSwarmUI uses title to recognise inputs](https://github.com/Stability-AI/StableSwarmUI/discussions/11#discussioncomment-6614931)
- [Mental Diffusion requires modifying the JSON workflow to identify inputs](https://github.com/nimadez/mental-diffusion#custom-workflows)

If there's a way to specify node level metadata(key value pairs), then it would be easy for third-party tools to integrate easily with ComfyUI. With such custom metadata, StableSwarmUI and MetalDiffusion could look for tool specific custom metadata such as `stable_swarm_input:true` or `mental_diffusion_input:true`. Instead of just true/false, the values could even be `checkpoint/seed/int/string/etc.` to support arbitrary integrations.