osbuild2/grub2: add unified option for uefi

The `uefi.unified` option indicates whether the `org.osbuild.grub2`
will use the unified grub configration scheme[1] used by Fedora 34
and thus RHEL 9.
NB: This requires osbuild version >= 32.

[1] https://fedoraproject.org/wiki/Changes/UnifyGrubConfig

internal/osbuild2/grub2_stage.go

@@ -22,6 +22,7 @@ type GRUB2StageOptions struct {
 type GRUB2UEFI struct {
 	Vendor  string `json:"vendor"`
 	Install bool   `json:"install,omitempty"`
+	Unified bool   `json:"unified,omitempty"`
 }
 
 func (GRUB2StageOptions) isStageOptions() {}