Enable the cloud-init service. This services reads configuration metadata in a cloud environment and configures the machine according to this metadata.
This configuration is not completely compatible with the NixOS way of doing configuration, as configuration done by cloud-init might be overridden by a subsequent nixos-rebuild call. However, some parts of cloud-init fall outside of NixOS's responsibility, like filesystem resizing and ssh public key provisioning, and cloud-init is useful for that parts. Thus, be wary that using cloud-init in NixOS might come as some cost.