Add grpcxx to grpc_bench

[Tradias]

For performance comparison with other implementations it would be nice to use an established benchmarking environment like https://github.com/LesnyRumcajs/grpc_bench. They accept merge requests. I have added my asio-grpc library in the past with no issues. Check out the existing cpp_ benchmark to get an idea on how to write the benchmark code.

[uatuko]

Thanks @Tradias, definitely something to look into.