Set default differ for the default unpack config of transfer service

Signed-off-by: Henry Wang <henwang@amazon.com>

defaults/defaults_linux.go

@@ -33,4 +33,6 @@ const (
 	// DefaultStateDir is the default location used by containerd to store
 	// transient data
 	DefaultStateDir = "/run/containerd"
+	// DefaultDiffer will set the default differ for the platform.
+	DefaultDiffer = "walking"
 )

defaults/defaults_unix_nolinux.go

@@ -33,4 +33,6 @@ const (
 	// DefaultStateDir is the default location used by containerd to store
 	// transient data
 	DefaultStateDir = "/var/run/containerd"
+	// DefaultDiffer will set the default differ for the platform.
+	DefaultDiffer = "walking"
 )

plugins/transfer/plugin_defaults_other.go

@@ -28,6 +28,7 @@ func defaultUnpackConfig() []unpackConfiguration {
 		{
 			Platform:    platforms.Format(platforms.DefaultSpec()),
 			Snapshotter: defaults.DefaultSnapshotter,
+			Differ:      defaults.DefaultDiffer,
 		},
 	}
 }