client: add Import() and Export() for importing/exporting image in OCI format

Export as a tar (Note: "-" can be used for stdout):

    $ ctr images export /tmp/oci-busybox.tar docker.io/library/busybox:latest

Import a tar (Note: "-" can be used for stdin):

    $ ctr images import foo/new:latest /tmp/oci-busybox.tar

Note: media types are not converted at the moment: e.g.
  application/vnd.docker.image.rootfs.diff.tar.gzip
  -> application/vnd.oci.image.layer.v1.tar+gzip

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

content/content.go

@@ -5,6 +5,7 @@ import (
 	"io"
 	"time"
 
+	"github.com/containerd/containerd/oci"
 	"github.com/opencontainers/go-digest"
 )
 
@@ -77,10 +78,8 @@ type IngestManager interface {
 }
 
 type Writer interface {
-	io.WriteCloser
+	oci.BlobWriter
 	Status() (Status, error)
-	Digest() digest.Digest
-	Commit(size int64, expected digest.Digest) error
 	Truncate(size int64) error
 }