importer: refactor

- Use lease API (previoisly, GC was not supported)
- Refactored interfaces for ease of future Docker v1 importer support

For usage, please refer to `ctr images import --help`.

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

images/importexport.go

@@ -0,0 +1,21 @@
+package images
+
+import (
+	"context"
+	"io"
+
+	"github.com/containerd/containerd/content"
+	ocispec "github.com/opencontainers/image-spec/specs-go/v1"
+)
+
+// Importer is the interface for image importer.
+type Importer interface {
+	// Import imports an image from a tar stream.
+	Import(ctx context.Context, store content.Store, reader io.Reader) ([]Image, error)
+}
+
+// Exporter is the interface for image exporter.
+type Exporter interface {
+	// Export exports an image to a tar stream.
+	Export(ctx context.Context, store content.Store, desc ocispec.Descriptor, writer io.Writer) error
+}