compression: add support for the zstd algorithm

zstd is a compression algorithm that has a very fast decoder, while
providing also good compression ratios.  The fast decoder makes it
suitable for container images, as decompressing the tarballs is a very
expensive operation.

https://github.com/opencontainers/image-spec/pull/788 added support
for zstd to the OCI image specs.

Signed-off-by: Giuseppe Scrivano <gscrivan@redhat.com>

images/mediatypes.go

@@ -78,6 +78,8 @@ func DiffCompression(ctx context.Context, mediaType string) (string, error) {
 			switch ext[len(ext)-1] {
 			case "gzip":
 				return "gzip", nil
+			case "zstd":
+				return "zstd", nil
 			}
 		}
 		return "", nil