server/discovery: add discovery service

server/discovery/internal/testservices/helloworld.proto

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 syntax = "proto3";
 
-package prpc;
+package testservices;
 
 // The greeting service definition.
 service Greeter {
   // Sends a greeting
   rpc SayHello (HelloRequest) returns (HelloReply) {}
 }
 
 // The request message containing the user's name.
 message HelloRequest {
   string name = 1;
 }
 
 // The response message containing the greetings
 message HelloReply {
   string message = 1;
 }
 
 service Calc {
   rpc Multiply (MultiplyRequest) returns (MultiplyResponse) {}
 }
 
 message MultiplyRequest {
   int32 x = 1;
   int32 y = 2;
 }
 
 message MultiplyResponse {
   int32 z = 1;
 }