Update differ to handle zstd media types

The differ should be able to generate zstd compressed layers when
provided with the zstd media type.

Signed-off-by: Derek McGowan <derek@mcg.dev>

pkg/archive/compression/compression.go

@@ -45,6 +45,8 @@ const (
 	Gzip
 	// Zstd is zstd compression algorithm.
 	Zstd
+	// Unknown is used when a plugin handles the algorithm.
+	Unknown
 )
 
 const (
@@ -257,6 +259,8 @@ func (compression *Compression) Extension() string {
 		return "gz"
 	case Zstd:
 		return "zst"
+	case Unknown:
+		return "unknown"
 	}
 	return ""
 }