cmd/protoc-gen-go-grpc: re-add proto3 'optional' support (#3914)

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

@@ -34,6 +34,7 @@ import (
 	"flag"
 
 	"google.golang.org/protobuf/compiler/protogen"
+	"google.golang.org/protobuf/types/pluginpb"
 )
 
 var requireUnimplemented *bool
@@ -45,6 +46,7 @@ func main() {
 	protogen.Options{
 		ParamFunc: flags.Set,
 	}.Run(func(gen *protogen.Plugin) error {
+		gen.SupportedFeatures = uint64(pluginpb.CodeGeneratorResponse_FEATURE_PROTO3_OPTIONAL)
 		for _, f := range gen.Files {
 			if !f.Generate {
 				continue