Adds `enable_secure_boot`, `enable_vtpm` and `enable_integrity_monitoring`
config options to enable building of custom Shielded GCP Compute images.
Feedback on this is more than welcome as this is my first attempt in
contributing to anything Packer related.
Packer is great for us to build custom images on top of GCP but we would
like to enhance that to support Shielded VM images. This will allow us
to have more secure and trusted images which our team(s) will be using.
