Support remote snapshotter through containerd-worker-snapshotter other than stargz

[liulanzheng]

What is the problem you're trying to solve

As containerd remote snapshotter allows users to implement their own remote snapshotters. But in buildkit, only stargz is supported for some judgment is hardcode to 'stargz'. And also some labels needed by remote snapshotters are not passed or no method provided to pass to snapshotters apis.

Describe the solution you'd like

The most important is pass the basic labels like "containerd.io/snapshot.ref" to containerd-worker-snapshotter preprare api other than startgz and handle the response. It is better to provide a method that users can pass their own labels, like the image reference or layer annotations.

[liulanzheng]

It is better to perform as the cri snapshot annotations, which pass image ref and annotations to snapshotter.

[antoinerg]

Is this still a current limitation?