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>

images/converter/uncompress/uncompress.go

@@ -0,0 +1,120 @@
+/*
+   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 uncompress
+
+import (
+	"compress/gzip"
+	"context"
+	"fmt"
+	"io"
+
+	"github.com/containerd/containerd/content"
+	"github.com/containerd/containerd/errdefs"
+	"github.com/containerd/containerd/images"
+	"github.com/containerd/containerd/images/converter"
+	"github.com/containerd/containerd/labels"
+	ocispec "github.com/opencontainers/image-spec/specs-go/v1"
+)
+
+var _ converter.ConvertFunc = LayerConvertFunc
+
+// LayerConvertFunc converts tar.gz layers into uncompressed tar layers.
+// Media type is changed, e.g., "application/vnd.oci.image.layer.v1.tar+gzip" -> "application/vnd.oci.image.layer.v1.tar"
+func LayerConvertFunc(ctx context.Context, cs content.Store, desc ocispec.Descriptor) (*ocispec.Descriptor, error) {
+	if !images.IsLayerType(desc.MediaType) || IsUncompressedType(desc.MediaType) {
+		// No conversion. No need to return an error here.
+		return nil, nil
+	}
+	info, err := cs.Info(ctx, desc.Digest)
+	if err != nil {
+		return nil, err
+	}
+	readerAt, err := cs.ReaderAt(ctx, desc)
+	if err != nil {
+		return nil, err
+	}
+	defer readerAt.Close()
+	sr := io.NewSectionReader(readerAt, 0, desc.Size)
+	newR, err := gzip.NewReader(sr)
+	if err != nil {
+		return nil, err
+	}
+	defer newR.Close()
+	ref := fmt.Sprintf("convert-uncompress-from-%s", desc.Digest)
+	w, err := cs.Writer(ctx, content.WithRef(ref))
+	if err != nil {
+		return nil, err
+	}
+	defer w.Close()
+
+	// Reset the writing position
+	// Old writer possibly remains without aborted
+	// (e.g. conversion interrupted by a signal)
+	if err := w.Truncate(0); err != nil {
+		return nil, err
+	}
+
+	n, err := io.Copy(w, newR)
+	if err != nil {
+		return nil, err
+	}
+	if err := newR.Close(); err != nil {
+		return nil, err
+	}
+	// no need to retain "containerd.io/uncompressed" label, but retain other labels ("containerd.io/distribution.source.*")
+	labelsMap := info.Labels
+	delete(labelsMap, labels.LabelUncompressed)
+	if err = w.Commit(ctx, 0, "", content.WithLabels(labelsMap)); err != nil && !errdefs.IsAlreadyExists(err) {
+		return nil, err
+	}
+	if err := w.Close(); err != nil {
+		return nil, err
+	}
+	newDesc := desc
+	newDesc.Digest = w.Digest()
+	newDesc.Size = n
+	newDesc.MediaType = convertMediaType(newDesc.MediaType)
+	return &newDesc, nil
+}
+
+func IsUncompressedType(mt string) bool {
+	switch mt {
+	case
+		images.MediaTypeDockerSchema2Layer,
+		images.MediaTypeDockerSchema2LayerForeign,
+		ocispec.MediaTypeImageLayer,
+		ocispec.MediaTypeImageLayerNonDistributable:
+		return true
+	default:
+		return false
+	}
+}
+
+func convertMediaType(mt string) string {
+	switch mt {
+	case images.MediaTypeDockerSchema2LayerGzip:
+		return images.MediaTypeDockerSchema2Layer
+	case images.MediaTypeDockerSchema2LayerForeignGzip:
+		return images.MediaTypeDockerSchema2LayerForeign
+	case ocispec.MediaTypeImageLayerGzip:
+		return ocispec.MediaTypeImageLayer
+	case ocispec.MediaTypeImageLayerNonDistributableGzip:
+		return ocispec.MediaTypeImageLayerNonDistributable
+	default:
+		return mt
+	}
+}