protoc-gen-{grpc-gateway,openapiv2}: add optional support

Let `protoc` know that these generators support optional keywords. This
is by no means an implementation in `grpc-gateway` of the optional aware
generated code, it's just shielding of being considered as blockers in
code generation workflows.

Closes grpc-ecosystem#1278

protoc-gen-grpc-gateway/main.go

@@ -18,6 +18,7 @@ import (
 	"github.com/grpc-ecosystem/grpc-gateway/v2/internal/descriptor"
 	"github.com/grpc-ecosystem/grpc-gateway/v2/protoc-gen-grpc-gateway/internal/gengateway"
 	"google.golang.org/protobuf/compiler/protogen"
+	"google.golang.org/protobuf/types/pluginpb"
 )
 
 var (
@@ -54,6 +55,7 @@ func main() {
 	protogen.Options{
 		ParamFunc: flag.CommandLine.Set,
 	}.Run(func(gen *protogen.Plugin) error {
+		gen.SupportedFeatures = uint64(pluginpb.CodeGeneratorResponse_FEATURE_PROTO3_OPTIONAL)
 		reg := descriptor.NewRegistry()
 
 		err := applyFlags(reg)

protoc-gen-openapiv2/main.go

@@ -146,7 +146,10 @@ func emitFiles(out []*descriptor.ResponseFile) {
 	for idx, item := range out {
 		files[idx] = item.CodeGeneratorResponse_File
 	}
-	emitResp(&pluginpb.CodeGeneratorResponse{File: files})
+	emitResp(&pluginpb.CodeGeneratorResponse{
+		SupportedFeatures: uint64(pluginpb.CodeGeneratorResponse_FEATURE_PROTO3_OPTIONAL),
+		File: files,
+	})
 }
 
 func emitError(err error) {