Add service plugin and support in process integration.

Signed-off-by: Lantao Liu <lantaol@google.com>

plugin/plugin.go

@@ -58,6 +58,8 @@ const (
 	AllPlugins Type = "*"
 	// RuntimePlugin implements a runtime
 	RuntimePlugin Type = "io.containerd.runtime.v1"
+	// ServicePlugin implements a internal service
+	ServicePlugin Type = "io.containerd.service.v1"
 	// GRPCPlugin implements a grpc service
 	GRPCPlugin Type = "io.containerd.grpc.v1"
 	// SnapshotPlugin implements a snapshotter