Benchmark purerpc under CPython/PyPy against other gRPC implementations

[standy66]

There is some benchmarking code in misc/greeter directory, notably test_perf.py, run_h2load.sh, latency_h2load.sh, but it needs to be refined and independently tested with other gRPC implementations, not limited to Python. Also this needs to be integrated in CI pipeline in some way, maybe similar to speed.python.org

Some useful links I found: bojand/ghz, gRPC official guide to benchmarking, performance dashboard