Support comparing multi-node image vs single-node image

[AkihiroSuda]

Currently the comparison fails before comparing the actual layers

$ docker build -t my-golang-1.21-alpine3.18 'https://github.com/docker-library/golang.git#master:1.21/alpine3.18'
$ diffoci diff golang:1.21-alpine3.18 docker://my-golang-1.21-alpine3.18 --verbose --semantic
INFO[0000] Loading image "docker.io/library/my-golang-1.21-alpine3.18:latest" from "docker" 
docker.io/library/my golang 1.21 alpine3	saved	
Importing	elapsed: 1.1 s	total:   0.0 B	(0.0 B/s)	
Event: "DescriptorMismatch"
  &v1.Descriptor{
  	MediaType: strings.Join({
  		"application/vnd.docker.distribution.manifest.",
- 		"list.",
  		"v2+json",
  	}, ""),
  	... // 4 ignored and 3 identical fields
  }

Event: "DescriptorMismatch" (field "Annotations")
  map[string]string(
- 	nil,
+ 	{
+ 		"io.containerd.image.name":          "docker.io/library/my-golang-1.21-alpine3.18:latest",
+ 		"org.opencontainers.image.ref.name": "latest",
+ 	},
  )

Event: "IndexBlobMismatch"
  v1.Index{
  	Versioned: {SchemaVersion: 2},
  	MediaType: strings.Join({
  		"application/vnd.docker.distribution.manifest.",
- 		"list.",
  		"v2+json",
  	}, ""),
  	ArtifactType: "",
  	... // 3 ignored fields
  }

Event: "IndexBlobMismatch" (field "Manifests": length mismatch)

[AkihiroSuda]

Experimentally implemented in #6