feat: support optional fields in proto v3

[kei2100]

refs #32

Add support optional fields in proto v3

e.g.

// test/foo.pb

message FooMessage {
  optional string string_val = 1;
}

$ protoc -I. -Itest/ --marshal-zap_out=test/ --go_out=test/ test/foo.proto                                                                                                                  

before

protoc failed
got the following error message

test/foo.proto: is a proto3 file that contains optional fields, but code generator protoc-gen-marshal-zap hasn't been updated to support optional fields in proto3. Please ask the owner of this code generator to support proto3 optional.--marshal-zap_out: 

after

protoc succeeded

// foo.pb.marshal-zap.go

func (x *FooMessage) MarshalLogObject(enc zapcore.ObjectEncoder) error {

	if x.StringVal != nil {
		enc.AddString("string_val", x.GetStringVal())
	}

	return nil
}