[Feature Request] support exporting nydus image to tarball

[imeoer]

Support exporting a nydus image to tarball file, to allow transfer nydus image between network-isolated environments.

The possible usage:

# on host A:
nydusify convert --source ubuntu:22.04 --target /path/to/ubuntu_nydus_22.04.gzip
scp /path/to/ubuntu_nydus_22.04.gzip $host-b:/path/to/ubuntu_nydus_22.04.gzip

# on host B:
nydusify copy --source /path/to/ubuntu_nydus_22.04.gzip --target registry.com/ubuntu:22.04_nydus