transfer: add OCI descriptors to Progress structure

The index media type and the digest are now shown in `ctr images pull --local=false`

Signed-off-by: Akihiro Suda <akihiro.suda.cz@hco.ntt.co.jp>

pkg/transfer/transfer.go

@@ -132,5 +132,5 @@ type Progress struct {
 	Parents  []string
 	Progress int64
 	Total    int64
-	// Descriptor?
+	Desc     *ocispec.Descriptor // since containerd v2.0
 }