Embed "Unimplemented" structs as proto recommended

Embedding these structs will be mandatory after migratring off from
gogo/protobuf (see #6564).

Signed-off-by: Kazuyoshi Kato <katokazu@amazon.com>

contrib/snapshotservice/service.go

@@ -32,6 +32,7 @@ var empty = &ptypes.Empty{}
 
 type service struct {
 	sn snapshots.Snapshotter
+	snapshotsapi.UnimplementedSnapshotsServer
 }
 
 // FromSnapshotter returns a Snapshot API server from a containerd snapshotter

services/containers/service.go

@@ -55,6 +55,7 @@ func init() {
 
 type service struct {
 	local api.ContainersClient
+	api.UnimplementedContainersServer
 }
 
 var _ api.ContainersServer = &service{}

services/content/contentserver/contentserver.go

@@ -38,6 +38,7 @@ import (
 
 type service struct {
 	store content.Store
+	api.UnimplementedContentServer
 }
 
 var bufPool = sync.Pool{

services/diff/service.go

@@ -53,6 +53,7 @@ func init() {
 
 type service struct {
 	local diffapi.DiffClient
+	diffapi.UnimplementedDiffServer
 }
 
 var _ diffapi.DiffServer = &service{}

services/events/service.go

@@ -52,6 +52,7 @@ func init() {
 type service struct {
 	ttService *ttrpcService
 	events    *exchange.Exchange
+	api.UnimplementedEventsServer
 }
 
 // NewService returns the GRPC events server

services/images/service.go

@@ -54,6 +54,7 @@ func init() {
 
 type service struct {
 	local imagesapi.ImagesClient
+	imagesapi.UnimplementedImagesServer
 }
 
 var _ imagesapi.ImagesServer = &service{}

services/introspection/service.go

@@ -62,6 +62,7 @@ func init() {
 
 type server struct {
 	local api.IntrospectionClient
+	api.UnimplementedIntrospectionServer
 }
 
 var _ = (api.IntrospectionServer)(&server{})

services/leases/service.go

@@ -47,6 +47,7 @@ func init() {
 
 type service struct {
 	lm leases.Manager
+	api.UnimplementedLeasesServer
 }
 
 func (s *service) Register(server *grpc.Server) error {

services/namespaces/service.go

@@ -54,6 +54,7 @@ func init() {
 
 type service struct {
 	local api.NamespacesClient
+	api.UnimplementedNamespacesServer
 }
 
 var _ api.NamespacesServer = &service{}

services/sandbox/controller_service.go

@@ -59,6 +59,7 @@ func init() {
 
 type controllerService struct {
 	local api.ControllerClient
+	api.UnimplementedControllerServer
 }
 
 var _ api.ControllerServer = (*controllerService)(nil)

services/sandbox/store_service.go

@@ -55,6 +55,7 @@ func init() {
 
 type sandboxService struct {
 	local api.StoreClient
+	api.UnimplementedStoreServer
 }
 
 var _ api.StoreServer = (*sandboxService)(nil)

services/snapshots/service.go

@@ -48,6 +48,7 @@ var empty = &ptypes.Empty{}
 
 type service struct {
 	ss map[string]snapshots.Snapshotter
+	snapshotsapi.UnimplementedSnapshotsServer
 }
 
 func newService(ic *plugin.InitContext) (interface{}, error) {

services/tasks/service.go

@@ -58,6 +58,7 @@ func init() {
 
 type service struct {
 	local api.TasksClient
+	api.UnimplementedTasksServer
 }
 
 func (s *service) Register(server *grpc.Server) error {

services/version/service.go

@@ -41,6 +41,7 @@ func initFunc(ic *plugin.InitContext) (interface{}, error) {
 }
 
 type service struct {
+	api.UnimplementedVersionServer
 }
 
 func (s *service) Register(server *grpc.Server) error {