add Image content converter

Go example:
```go
opts := []converter.Opt{
  // convert Docker media types to OCI ones
  converter.WithDocker2OCI(true),
  // convert tar.gz layers to uncompressed tar layers
  converter.WithLayerConvertFunc(uncompress.LayerConvertFunc),
}
srcRef := "example.com/foo:orig"
dstRef := "example.com/foo:converted"
dstImg, err = converter.Convert(ctx, client, dstRef, srcRef, opts...)
fmt.Println(dstImg.Target)
```

ctr example: `ctr images convert --oci --uncompress example.com/foo:orig example.com/foo:converted`

Go test: `go test -exec sudo -test.root -test.run TestConvert`

The implementation is from https://github.com/containerd/stargz-snapshotter/pull/224,
but eStargz-specific functions are not included in this PR.

eStargz converter can be specified by importing `estargz` package and using `WithLayerConvertFunc(estargz.LayerConvertFunc)` option.

This converter interface will be potentially useful for converting zstd and ocicrypt layers as well.

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

cmd/ctr/commands/images/convert.go

@@ -0,0 +1,108 @@
+/*
+   Copyright The containerd Authors.
+
+   Licensed under the Apache License, Version 2.0 (the "License");
+   you may not use this file except in compliance with the License.
+   You may obtain a copy of the License at
+
+       http://www.apache.org/licenses/LICENSE-2.0
+
+   Unless required by applicable law or agreed to in writing, software
+   distributed under the License is distributed on an "AS IS" BASIS,
+   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+   See the License for the specific language governing permissions and
+   limitations under the License.
+*/
+
+package images
+
+import (
+	"fmt"
+
+	"github.com/containerd/containerd/cmd/ctr/commands"
+	"github.com/containerd/containerd/images/converter"
+	"github.com/containerd/containerd/images/converter/uncompress"
+	"github.com/containerd/containerd/platforms"
+	ocispec "github.com/opencontainers/image-spec/specs-go/v1"
+	"github.com/pkg/errors"
+	"github.com/urfave/cli"
+)
+
+var convertCommand = cli.Command{
+	Name:      "convert",
+	Usage:     "convert an image",
+	ArgsUsage: "[flags] <source_ref> <target_ref>",
+	Description: `Convert an image format.
+
+e.g., 'ctr convert --uncompress --oci example.com/foo:orig example.com/foo:converted'
+
+Use '--platform' to define the output platform.
+When '--all-platforms' is given all images in a manifest list must be available.
+`,
+	Flags: []cli.Flag{
+		// generic flags
+		cli.BoolFlag{
+			Name:  "uncompress",
+			Usage: "convert tar.gz layers to uncompressed tar layers",
+		},
+		cli.BoolFlag{
+			Name:  "oci",
+			Usage: "convert Docker media types to OCI media types",
+		},
+		// platform flags
+		cli.StringSliceFlag{
+			Name:  "platform",
+			Usage: "Pull content from a specific platform",
+			Value: &cli.StringSlice{},
+		},
+		cli.BoolFlag{
+			Name:  "all-platforms",
+			Usage: "exports content from all platforms",
+		},
+	},
+	Action: func(context *cli.Context) error {
+		var convertOpts []converter.Opt
+		srcRef := context.Args().Get(0)
+		targetRef := context.Args().Get(1)
+		if srcRef == "" || targetRef == "" {
+			return errors.New("src and target image need to be specified")
+		}
+
+		if !context.Bool("all-platforms") {
+			if pss := context.StringSlice("platform"); len(pss) > 0 {
+				var all []ocispec.Platform
+				for _, ps := range pss {
+					p, err := platforms.Parse(ps)
+					if err != nil {
+						return errors.Wrapf(err, "invalid platform %q", ps)
+					}
+					all = append(all, p)
+				}
+				convertOpts = append(convertOpts, converter.WithPlatform(platforms.Ordered(all...)))
+			} else {
+				convertOpts = append(convertOpts, converter.WithPlatform(platforms.DefaultStrict()))
+			}
+		}
+
+		if context.Bool("uncompress") {
+			convertOpts = append(convertOpts, converter.WithLayerConvertFunc(uncompress.LayerConvertFunc))
+		}
+
+		if context.Bool("oci") {
+			convertOpts = append(convertOpts, converter.WithDockerToOCI(true))
+		}
+
+		client, ctx, cancel, err := commands.NewClient(context)
+		if err != nil {
+			return err
+		}
+		defer cancel()
+
+		newImg, err := converter.Convert(ctx, client, targetRef, srcRef, convertOpts...)
+		if err != nil {
+			return err
+		}
+		fmt.Fprintln(context.App.Writer, newImg.Target.Digest.String())
+		return nil
+	},
+}

cmd/ctr/commands/images/images.go

@@ -50,6 +50,7 @@ var Command = cli.Command{
 		removeCommand,
 		tagCommand,
 		setLabelsCommand,
+		convertCommand,
 	},
 }