Consolidate gogo/protobuf dependencies under our own protobuf package

This would make gogo/protobuf migration easier.

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

protobuf/types/types.go

@@ -0,0 +1,26 @@
+/*
+   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 types provides convinient aliases that make google.golang.org/protobuf migration easier.
+package types
+
+import (
+	gogo "github.com/gogo/protobuf/types"
+)
+
+type Empty = gogo.Empty
+type Any = gogo.Any
+type FieldMask = gogo.FieldMask