compiler: support proto3 field presence for codegen
compiler: support proto3 field presence for codegen

Support proto3 field presence #7051.

This is the same change as https://github.com/grpc/grpc/pull/22998/files#diff-6c29ff8771ae54ddf2c14e2ef3eb554fR37-R44
grpc/grpc-java,dapengzhang0/grpc-java,stanley-cheung/grpc-java,dapengzhang0/grpc-java,ejona86/grpc-java,ejona86/grpc-java,stanley-cheung/grpc-java,stanley-cheung/grpc-java,grpc/grpc-java,ejona86/grpc-java,grpc/grpc-java,dapengzhang0/grpc-java,ejona86/grpc-java,grpc/grpc-java,stanley-cheung/grpc-java,dapengzhang0/grpc-java