`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 <danailster@gmail.com>
