Add ContainerService.Info to get info about a single container

For clients which only want to know about one container this is simpler than
searching the result of execution.List.

Signed-off-by: Ian Campbell <ian.campbell@docker.com>

cmd/ctr/main.go

@@ -44,6 +44,7 @@ containerd client
 		eventsCommand,
 		deleteCommand,
 		listCommand,
+		infoCommand,
 		shimCommand,
 		pprofCommand,
 	}