Add support for squashfs images in oci via atomfs

Add support for squashfs images in oci via atomfs

This adds support to the oci template for squashfs images.
It uses 'atomfs' from [1] to accomplish this.

Squashfs images (media type
application/vnd.stacker.image.layer.squashfs+zstd+verity) have several
benefits compared to tar+gz:

 * immediately mountable
 * read-only filesystem
 * verity data present in oci manifest.

I presented this at Fosdem 2023 at [2].

The 'atomfs' program can be replaced by passing '--mount-helper'
argument to the oci template.

    mount-helper mount oci:<oci_dir>:<oci_name> <mountpoint>
    mount-helper umount <mountpoint>

[1] https://github.com/project-machine/atomfs
[2] https://fosdem.org/2023/schedule/event/container_secure_storage/

Signed-off-by: Scott Moser <smoser@brickies.net>