Add protoc-gen-go-fieldpath

This command will replace protobuf/plugin/fieldpath when we migrate
off from gogo/protobuf.

See #6564 for the detail.

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

cmd/protoc-gen-go-fieldpath/main.go

@@ -0,0 +1,35 @@
+/*
+   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 main
+
+import (
+	"google.golang.org/protobuf/compiler/protogen"
+)
+
+func main() {
+	protogen.Options{}.Run(func(gen *protogen.Plugin) error {
+		for _, f := range gen.Files {
+			if !f.Generate {
+				continue
+			}
+			if err := generate(gen, f); err != nil {
+				return err
+			}
+		}
+		return nil
+	})
+}