[Feat] Support reconvert image from nydus to oci

[basuotian]

Feat: dragonflyoss/nydus#1682

The current implement is very rough as some logic should be changed in "github.com/containerd/containerd/v2/core/images/converter". (see unit test testImageReConvertBasic)

Please take a look and see if is necessary to open a pr to containerd.

[imeoer]

@basuotian Thanks for the PR! How about moving the containerdreconverter codes into a forked nydus-snapshotter repo? This way we can clearly see the diff of the changes. :)

[basuotian]

@basuotian Thanks for the PR! How about moving the containerdreconverter codes into a forked nydus-snapshotter repo? This way we can clearly see the diff of the changes. :)

OK, I did move containerdreconverter into repo github.com/basuotian/nydus-snapshotter-reconverter, and these codes should be part of github.com/containerd/containerd/v2/core/images. @imeoer PTAL