add flag `-output-request-as-grpcurl`

[jschwinger233]

I was wondering if it's a good idea to add a flag -output-request-as-grpcurl, which will serve as the similar function to Chrome's copy as cURL.

I believe this may well help developers further more, but there is a concern about defying of Unix philosophy "Make each program do one thing well"...

For you reference, grpcurl is grpc's version of cURL, allowing us to send grpc requests from command line, and here's a typical use case:

grpcurl -plaintext  -proto rpc/gen/core.proto -d '{"name":"eru"}'   localhost:5001 pb.CoreRPC/GetPod