Add an RPC echo example

[Romero027]

Why are these changes needed?

Created a new example for the upcoming workshop demo

[Romero027]

cc @crazyboycjr

[crazyboycjr]

Maybe we can merge rpc_hello and rpc_echo into one. I think rpc_echo is used widely in gRPC's examples, so we can use rpc_echo as the name. Under this example, we have client.rs, server.rs, and frontend.rs, and a README.md. You can also update all reference to rpc_hello and to rpc_echo in the doc and README accordingly.

[crazyboycjr]

LGTM, thanks!