Introduce plugins/list subcommand
Introduce plugins/list subcommand

`Ctr` interface follows the pattern `ctr <command> <subcommand>` except
for the `plugins` command which does not have subcommands. This feels
unnatural to certain users and they would expect that they can list
containerd plugins via `ctr plugins list`.

This commit implements their expectation so that `plugins` becomes a
command "group" and its `list` subcommand actually lists the plugins.

Signed-off-by: Danail Branekov <43f82d874965b9fe2876a1a864fffde1e64d6143@gmail.com>

kunalkushwaha/containerd,estesp/containerd,kunalkushwaha/containerd,containerd/containerd,vdemeester/containerd,containerd/containerd,dmcgowan/containerd,thaJeztah/containerd,mikebrow/containerd,miaoyq/containerd,crosbymichael/containerd,thaJeztah/containerd,estesp/containerd,crosbymichael/containerd,dmcgowan/containerd,miaoyq/containerd,mikebrow/containerd,vdemeester/containerd