Support flattened convert in userspace convertor

### What is the version of your Accelerated Container Image

_No response_

### What would you like to be added?

Support generating a single layer overlaybd image in userspace convertor

### Why is this needed for Accelerated Container Image?

Our production images have many layers (~70) and we're not using [`Layer/Manifest Deduplication`](https://github.com/containerd/accelerated-container-image/blob/main/docs/USERSPACE_CONVERTOR.md).

Sometimes we observe long latency during rpull, and the latency is proportional to the image layers. (We suspect it's due to network issue when containerd/overlaybd talks to cloud provider during rpull to get layer metadata).

Currently our mitigation is to first [flatten/squash](https://github.com/google/go-containerregistry/blob/main/cmd/crane/doc/crane_flatten.md) the original image into a single layer image, and then convert that single layer image. This solves the long rpull issue.

It would be great if the userspace convertor can provide a feature that generates a single-layer overlaybd image directly.

cc @shuaichang

### Are you willing to submit PRs to contribute to this feature?

- [ ] Yes, I am willing to implement it.