use proto3 idl to generate grpc plugin code

[ahfuzhang]

1.add a tool to generate code: examples/code_generator/cmd/code_gen.go
2.add a example to show how to use proto3 to define plugin service: examples/code_generator/proto/my_test_grpc_plugin.proto
3.add readme file to explain how to use this tool: examples/code_generator/readme.md

thanks

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