Consolidate gogo/protobuf dependencies under our own protobuf package

This would make gogo/protobuf migration easier.

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

integration/client/client_ttrpc_test.go

@@ -25,8 +25,8 @@ import (
 	"github.com/containerd/containerd/namespaces"
 	"github.com/containerd/containerd/pkg/ttrpcutil"
 	"github.com/containerd/containerd/protobuf"
+	"github.com/containerd/containerd/protobuf/types"
 	"github.com/containerd/ttrpc"
-	"github.com/gogo/protobuf/types"
 	"github.com/stretchr/testify/assert"
 )