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>

convert_test.go

@@ -0,0 +1,88 @@
+/*
+   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 containerd
+
+import (
+	"testing"
+
+	"github.com/containerd/containerd/images"
+	"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"
+	"gotest.tools/v3/assert"
+)
+
+// TestConvert creates an image from testImage, with the following conversion:
+// - Media type: Docker -> OCI
+// - Layer type: tar.gz -> tar
+// - Arch:       Multi  -> Single
+func TestConvert(t *testing.T) {
+	if testing.Short() {
+		t.Skip()
+	}
+	ctx, cancel := testContext(t)
+	defer cancel()
+
+	client, err := New(address)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer client.Close()
+
+	_, err = client.Fetch(ctx, testImage)
+	if err != nil {
+		t.Fatal(err)
+	}
+	dstRef := testImage + "-testconvert"
+	defPlat := platforms.DefaultStrict()
+	opts := []converter.Opt{
+		converter.WithDockerToOCI(true),
+		converter.WithLayerConvertFunc(uncompress.LayerConvertFunc),
+		converter.WithPlatform(defPlat),
+	}
+	dstImg, err := converter.Convert(ctx, client, dstRef, testImage, opts...)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer func() {
+		if deleteErr := client.ImageService().Delete(ctx, dstRef); deleteErr != nil {
+			t.Fatal(deleteErr)
+		}
+	}()
+	cs := client.ContentStore()
+	plats, err := images.Platforms(ctx, cs, dstImg.Target)
+	if err != nil {
+		t.Fatal(err)
+	}
+	// Assert that the image does not have any extra arch.
+	assert.Equal(t, 1, len(plats))
+	assert.Check(t, defPlat.Match(plats[0]))
+
+	// Assert that the media type is converted to OCI and also uncompressed
+	mani, err := images.Manifest(ctx, cs, dstImg.Target, defPlat)
+	if err != nil {
+		t.Fatal(err)
+	}
+	for _, l := range mani.Layers {
+		if plats[0].OS == "windows" {
+			assert.Equal(t, ocispec.MediaTypeImageLayerNonDistributable, l.MediaType)
+		} else {
+			assert.Equal(t, ocispec.MediaTypeImageLayer, l.MediaType)
+		}
+	}
+}