Make nydus-snapshotter capable to download artifacts itself.

[changweige]

We currently see more and more needs to let nydus-snapshotter download OCI artifacts on its own.

1. Nydus-snapshotter can't start nydusd if containerd clients like ctr and nerdtcl are only pulling image rather than running a container. It's because nydus-snapshotter can only know where the meta file (bootstrap) is located after the Prepare operation for it. So nydus-snapshotter must wait for the wriable layer on top of it.

2. Container download container image, decompress and untar it in an atomic way, which means nydus-snapshotter can't handle the tar files itself to eliminate the unnecessary untar operations.

Nydus-snapshotter's configuration on mirror hosts is already compatible with container'd, which means nydus-snapshotter can directly use use some of containerd's public packages and have capability to download artifacts from mirror registry.