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>

vendor/github.com/containerd/cgroups/control.go

@@ -82,6 +82,9 @@ type Cgroup interface {
 	Thaw() error
 	// OOMEventFD returns the memory subsystem's event fd for OOM events
 	OOMEventFD() (uintptr, error)
+	// RegisterMemoryEvent returns the memory subsystems event fd for whatever memory event was
+	// registered for. Can alternatively register for the oom event with this method.
+	RegisterMemoryEvent(MemoryEvent) (uintptr, error)
 	// State returns the cgroups current state
 	State() State
 	// Subsystems returns all the subsystems in the cgroup