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>

services/diff/service.go

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