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>

export_test.go

@@ -0,0 +1,64 @@
+package containerd
+
+import (
+	"archive/tar"
+	"io"
+	"runtime"
+	"testing"
+)
+
+// TestExport exports testImage as a tar stream
+func TestExport(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)
+	}
+
+	exportedStream, err := client.Export(ctx, pulled.Target())
+	if err != nil {
+		t.Fatal(err)
+	}
+	assertOCITar(t, exportedStream)
+}
+
+func assertOCITar(t *testing.T, r io.Reader) {
+	// TODO: add more assertion
+	tr := tar.NewReader(r)
+	foundOCILayout := false
+	foundIndexJSON := false
+	for {
+		h, err := tr.Next()
+		if err == io.EOF {
+			break
+		}
+		if err != nil {
+			t.Error(err)
+			continue
+		}
+		if h.Name == "oci-layout" {
+			foundOCILayout = true
+		}
+		if h.Name == "index.json" {
+			foundIndexJSON = true
+		}
+	}
+	if !foundOCILayout {
+		t.Error("oci-layout not found")
+	}
+	if !foundIndexJSON {
+		t.Error("index.json not found")
+	}
+}