Add partman support for Discoverable Partitions Spec

The Discoverable Partitions Specification defines a bunch of GUIDs that allow auto-discovering the mount point of a GPT partition on boot. This for example can be used to boot an image using a dracut or package based initrd without the need of manually defining and maintaining /etc/fstab.

https://uapi-group.org/specifications/specs/discoverable_partitions_specification/

Enable this unconditionally on Linux as there are no downsides, tools that use this spec will use it, and those that don't will ignore it, as the current default is the generic "Linux data" GUID which is the same across all partitions.