Include stargz snapshotter (and make the default?)

[nakamorichi]

Problem Description

It seems stargz snapshotter is not available when running RD in non-k8s containerd mode.

Proposed Solution

Set stargz up as proxy plugin, and use it via socket in containerd mode regardless of k8s on or off. Also, unless there is reason for not having it as the default snapshotter, also make it the default.

Additionally, containerd configuration should be made persistently overridable, as currently the configuration is overwritten with each RD restart.

Additional Information

No response

[AkihiroSuda]

Cc @ktock (stargz-snapshotter maintainer)

[ktock]

SGTM 👍