imagetools: support oci-layout refs

Add oci-layout:// source and target support to imagetools create and
inspect while keeping merge, filter, and referrer logic shared.

Signed-off-by: Tonis Tiigi <tonistiigi@gmail.com>

util/imagetools/auth.go

@@ -15,6 +15,13 @@ func RegistryAuthForRef(ref string, auth authprovider.AuthConfigProvider) (strin
 	if auth == nil {
 		return "", nil
 	}
+	loc, err := ParseLocation(ref)
+	if err != nil {
+		return "", err
+	}
+	if loc.IsOCILayout() {
+		return "", nil
+	}
 	r, err := parseRef(ref)
 	if err != nil {
 		return "", err