image: add image kind abstraction

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.

internal/image/image.go

@@ -0,0 +1,29 @@
+package image
+
+import (
+	"math/rand"
+
+	"github.com/osbuild/osbuild-composer/internal/artifact"
+	"github.com/osbuild/osbuild-composer/internal/manifest"
+	"github.com/osbuild/osbuild-composer/internal/rpmmd"
+	"github.com/osbuild/osbuild-composer/internal/runner"
+)
+
+type ImageKind interface {
+	Name() string
+	InstantiateManifest(m *manifest.Manifest, repos []rpmmd.RepoConfig, runner runner.Runner, rng *rand.Rand) (*artifact.Artifact, error)
+}
+
+type Base struct {
+	name string
+}
+
+func (img Base) Name() string {
+	return img.name
+}
+
+func NewBase(name string) Base {
+	return Base{
+		name: name,
+	}
+}