question: support for v1layout tarball?

The [v1/tarball.Write()](https://pkg.go.dev/github.com/google/go-containerregistry/pkg/v1/tarball#Write) writes it in docker's unique tarball format. Per the summary:

> This package produces tarballs that can consumed via docker load. Note that this is a different format from the [legacy](https://github.com/google/go-containerregistry/blob/v0.19.0/pkg/v1/tarball/pkg/legacy/tarball) tarballs that are produced by docker save, but this package is still able to read the legacy tarballs produced by docker save.

As of docker engine v25, `docker save` actually writes to OCI layout tarball, i.e.:

```bash
$ docker save alpine:3.19 > /tmp/alpine319.tar
$ tar xvf /tmp/alpine319.tar
x blobs/
x blobs/sha256/
x blobs/sha256/a0264d60f80df12bc1e6dd98bae6c43debe6667c0ba482711f0d806493467a46
x blobs/sha256/ace17d5d883e9ea5a21138d0608d60aa2376c68f616c55b0b7e73fba6d8556a3
x blobs/sha256/bca4290a96390d7a6fc6f2f9929370d06f8dfcacba591c76e3d5c5044e7f420c
x blobs/sha256/c5b1261d6d3e43071626931fc004f70149baeba2c8ec672bd4f27761f8e1ad6b
x index.json
x manifest.json
x oci-layout
```

That is almost identical to v1 layout, which GGCR supports, with the exception of:

* it is in a tar stream, not an actual layout
* it includes docker's unique `manifest.json`

FWIW, this is identical to the results of using buildkit with `--output type=oci,path=/tmp/foo.tar`

Is there a way to output this tar format? In theory, we could output to a directory using `layout.Write()` and then tar it up, but that is inefficient and could get messed up, and in any case, it all is a tar stream.

Start with pkg/v1/tarball and the v1/tarball.Write entry point, then compare its output with pkg/v1/tarball/pkg/legacy/tarball and layout.Write. Check how Docker v25 and BuildKit structure OCI layout tar streams, including manifest.json; done means the library can produce that tar format without requiring an intermediate directory.

Written by the indexing model from the issue text.