I made this (minus NixOS support, I should add that) for proxmox VE using their API a few weeks ago. I mean it's not this extensive, but it works:
I mean, I'm not going to claim it's remotely near the same quality. And proxmox has some holes in their support for cloud init. And of course you need a mini pc on a good internet connection or the like.
But extremely fast provisioning of a any of VMs ... very handy.
Proxmox has too many compromises though. Maybe I should do the reverse, and extend this until it can fully replace proxmox entirely.