Implement all of Fedora in terms of this new abstraction. What used to be the
manifest functions (and before that the pipeline functions) are now the image
functions, whose purpose is to instantiate the right image kind structs from the
image type definitions we currently have in the distro definition.
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.
