The plugin protoc-gen-dubbo should support optional fields

[dixingxing0]

What would you like to be added:
protoc-gen-dubbo should support optional fields.

We run this command:
protoc -I ./ AdItemFeature.proto --dubbo_out=plugins=grpc+dubbo:.

and we meet this error:

/usr/bin/make -f /IdeaProjects/feature-service/protobuf/Makefile -C /IdeaProjects/feature-service/protobuf dubbo-gen
WARNING: Package "github.com/golang/protobuf/protoc-gen-go/generator" is deprecated.
  A future release of golang/protobuf will delete this package,
  which has long been excluded from the compatibility promise.

AdItemFeature.proto: is a proto3 file that contains optional fields, but code generator protoc-gen-dubbo hasn't been updated to support optional fields in proto3. Please ask the owner of this code generator to support proto3 optional.--dubbo_out: 
make: *** [dubbo-gen] Error

Why is this needed:
In our protobuffer definition, there are lot of optional fields, i think protoc-gen-dubbo should support them.

[dixingxing0]

Seems we don't need optional in proto3.
protocolbuffers/protobuf#2497