Configs with a `boot_device.luks.device` set but no method (tpm2, tang, etc.) set should fail validation

[travier]

This config currently passes validation but should fail:

$ cat config.by
variant: fcos
version: 1.6.0-experimental
boot_device:
  luks:
    device: /dev/sda

$ butane --strict --pretty < config.bu
{
  "ignition": {
    "version": "3.5.0-experimental"
  }
}

Related to: #536