Support priv/unpriv image extraction

Optionally add back privileged extraction which can be enabled with
LUET_PRIVILEGED_EXTRACT=true

Signed-off-by: Ettore Di Giacinto <mudler@sabayon.org>

vendor/github.com/moby/buildkit/exporter/exporter.go

@@ -0,0 +1,22 @@
+package exporter
+
+import (
+	"context"
+
+	"github.com/moby/buildkit/cache"
+)
+
+type Exporter interface {
+	Resolve(context.Context, map[string]string) (ExporterInstance, error)
+}
+
+type ExporterInstance interface {
+	Name() string
+	Export(context.Context, Source) (map[string]string, error)
+}
+
+type Source struct {
+	Ref      cache.ImmutableRef
+	Refs     map[string]cache.ImmutableRef
+	Metadata map[string][]byte
+}