This abstracts away the manifest instantiation. The idea is that we define one
of these image kind types to represent a group of image types that are
sufficiently similar. Each image kind will have a struct with with all the
properties that can be customised for the image and a function to turn that into
an actual manifest. This is similar to how distro/fedora/manifest.go and
cmd/osbuild-playground works today, and aspires to move these closer together
and to eventually make the distro definitions simpler.
For now cmd/osbuild-playground is moved over to using the new abstraction.
