Support non-protobuf message formats like Avro

[brandonros]

Send non-proto rpc

For using gRPC with protocols other than protobuf, you will need the exact method name string and a file containing the raw bytes to be sent on the wire.

$ grpc_cli call localhost:50051 /helloworld.Greeter/SayHello \
  --input_binary_file=input.bin \
  --output_binary_file=output.bin
On success, you will need to read or decode the response from the output.bin file.

https://github.com/grpc/grpc/blob/master/doc/command_line_tool.md

Your tool can't do this, right?