[ComposeFS Backend] Integration with GrubCC bootloader

[Johan-Liebert1]

GrubCC (Grub Confidential Clusters) is a minified Grub build which would work similar to systemd-boot. We would want to integrate this as another bootloader option for now. Later we should plan on removing all Grub related code and only support systemd-boot and GrubCC

Related
https://redhat.atlassian.net/browse/RHEL-171399