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

examples/internal/proto/examplepb/a_bit_of_everything.proto

@@ -279,6 +279,9 @@ message ABitOfEverything {
 
 	// mark a field as readonly in Open API definition
 	string output_only_string_via_field_behavior_annotation = 39 [(google.api.field_behavior) = OUTPUT_ONLY];
+
+	// mark a field as optional
+	optional string optionl_string_value = 40;
 }
 
 // ABitOfEverythingRepeated is used to validate repeated path parameter functionality

protoc-gen-openapiv2/main.go

@@ -146,7 +146,8 @@ func emitFiles(out []*descriptor.ResponseFile) {
 	for idx, item := range out {
 		files[idx] = item.CodeGeneratorResponse_File
 	}
-	emitResp(&pluginpb.CodeGeneratorResponse{File: files})
+	supportedFeatures := uint64(pluginpb.CodeGeneratorResponse_FEATURE_PROTO3_OPTIONAL)
+	emitResp(&pluginpb.CodeGeneratorResponse{SupportedFeatures: &supportedFeatures, File: files})
 }
 
 func emitError(err error) {