cgroup2: implement containerd.events.TaskOOM event

How to test (from https://github.com/opencontainers/runc/pull/2352#issuecomment-620834524):
  (host)$ sudo swapoff -a
  (host)$ sudo ctr run -t --rm --memory-limit $((1024*1024*32)) docker.io/library/alpine:latest foo
  (container)$ sh -c 'VAR=$(seq 1 100000000)'

An event `/tasks/oom {"container_id":"foo"}` will be displayed in `ctr events`.

Signed-off-by: Akihiro Suda <akihiro.suda.cz@hco.ntt.co.jp>

pkg/oom/oom.go

@@ -0,0 +1,30 @@
+// +build linux
+
+/*
+   Copyright The containerd Authors.
+
+   Licensed under the Apache License, Version 2.0 (the "License");
+   you may not use this file except in compliance with the License.
+   You may obtain a copy of the License at
+
+       http://www.apache.org/licenses/LICENSE-2.0
+
+   Unless required by applicable law or agreed to in writing, software
+   distributed under the License is distributed on an "AS IS" BASIS,
+   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+   See the License for the specific language governing permissions and
+   limitations under the License.
+*/
+
+package oom
+
+import (
+	"context"
+)
+
+// Watcher watches OOM events
+type Watcher interface {
+	Close() error
+	Run(ctx context.Context)
+	Add(id string, cg interface{}) error
+}