just gen cpp async or sync SourceCode [code_generator] [diy_code_generator]

[stan1334]

What language does this apply to?
If it's a proto syntax change, is it for proto2 or proto3?
If it's about generated code change, what programming language?

Describe the problem you are trying to solve.
i cannot find cmd params 2 set just code sync or async code

Describe the solution you'd like

protoc --grpc_out=./ --plugin=protoc-gen-grpc="/C:/Program Files/grpc/bin/grpc_cpp_plugin.exe sync" helloworld.proto
protoc --grpc_out=./ --plugin=protoc-gen-grpc="/C:/Program Files/grpc/bin/grpc_cpp_plugin.exe" --grpc_out=async helloworld.proto

Describe alternatives you've considered
add MYCppGrpcGenerator just like CppGrpcGenerator class.

Additional context
Add any other context or screenshots about the feature request here.

Administrator@CHINAMI-GJCV7IR MINGW64 /d/code/Tools/grpc/examples/protos (master)
$ protoc --plugin=protoc-gen-grpc="/C:/Program Files/grpc/bin/grpc_cpp_plugin.exe" --grpc_out=./ --cpp_out=./ --grpc_opt=generate_async=true helloworld.proto --grpc_out: helloworld.proto: Unknown parameter: generate_async=true