Move plugin context events into separate plugin

Signed-off-by: Derek McGowan <derek@mcg.dev>

cmd/containerd/builtins.go

@@ -19,6 +19,7 @@ package main
 // register containerd builtins here
 import (
 	_ "github.com/containerd/containerd/diff/walking/plugin"
+	_ "github.com/containerd/containerd/events/plugin"
 	_ "github.com/containerd/containerd/gc/scheduler"
 	_ "github.com/containerd/containerd/runtime/restart/monitor"
 	_ "github.com/containerd/containerd/services/containers"