Add service plugin and support in process integration.

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

services/services.go

@@ -0,0 +1,36 @@
+/*
+   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 services
+
+const (
+	// ContentService is id of content service.
+	ContentService = "content-service"
+	// SnapshotsService is id of snapshots service.
+	SnapshotsService = "snapshots-service"
+	// ImagesService is id of images service.
+	ImagesService = "images-service"
+	// ContainersService is id of containers service.
+	ContainersService = "containers-service"
+	// TasksService is id of tasks service.
+	TasksService = "tasks-service"
+	// NamespacesService is id of namespaces service.
+	NamespacesService = "namespaces-service"
+	// LeasesService is id of leases service.
+	LeasesService = "leases-service"
+	// DiffService is id of diff service.
+	DiffService = "diff-service"
+)