ComfyUI custom nodes

[lboklin]

Support for adding custom nodes to the comfyui derivation. This PR made an attempt at such an implementation: NixOS/nixpkgs#268378.

[BatteredBunny]

I personally don't really use custom nodes, and the one i use, comfyui manager i just patch in manually by adding required inputs and cloning the repo into custom_nodes folder. I noticed your nix-ai-stuff fork, if you manage to get it working properly (idk if you did already) feel free to make a PR. Though i feel like your PR in nixified-ai has more potential as it has an actual cachix repo as i understand, instead of me building everything manually here and pushing into it (which is extremely painful as you have probably noticed)