distro/rhel9: add compression option to imageType

The compression field defines the compression type for the final image,
if any.
This is a temporary change.  We should design a better way to specify
"transport compression" for images, compression that is not required by
the image type or platform, but simply done for transport convenience or
storage.

internal/distro/rhel9/imagetype.go

@@ -55,6 +55,7 @@ type imageType struct {
 	name               string
 	nameAliases        []string
 	filename           string
+	compression        string // TODO: remove from image definition and make it a transport option
 	mimeType           string
 	packageSets        map[string]packageSetFunc
 	packageSetChains   map[string][]string