Support layer propagation to registries for image conversion on the fly

[ktock]

Related to:

• #3731@containerd
• #5913@knative/serving

Currently, this remote snapshotter only supports reading layers. So we need to pre-process(e.g. stargzify) and push an image to a registry in advance to make it usable as "remote layers" from this snapshotter.

We can achieve on-the-fly image processing without pre-processing it by enabling to propagate layers from local to registries during runtime:

1. On 1st Prepare(), normal(not remote) layers are pulled in a very normal way.
2. On Commit(), remote snapshotter processes(e.g. stargzifies) this layer as needed and pushes it to the remote store.
3. Since 2nd Prepare(), we can pull this layer as a "remote layer".