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>

import_test.go

@@ -0,0 +1,44 @@
+package containerd
+
+import (
+	"runtime"
+	"testing"
+)
+
+// TestExportAndImport exports testImage as a tar stream,
+// and import the tar stream as a new image.
+func TestExportAndImport(t *testing.T) {
+	// TODO: support windows
+	if testing.Short() || runtime.GOOS == "windows" {
+		t.Skip()
+	}
+	ctx, cancel := testContext()
+	defer cancel()
+
+	client, err := New(address)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer client.Close()
+
+	pulled, err := client.Pull(ctx, testImage)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	exported, err := client.Export(ctx, pulled.Target())
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	importRef := "test/export-and-import:tmp"
+	_, err = client.Import(ctx, importRef, exported, WithRefObject("@"+pulled.Target().Digest.String()))
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	err = client.ImageService().Delete(ctx, importRef)
+	if err != nil {
+		t.Fatal(err)
+	}
+}