title: does grpc service must have exactly one input parameter and one return value
body: <p>let's say i have a proto file like this. can I define service like this</p>
  
  <pre><code>rpc SayHello () returns (Response) {} //service has no input
  rpc SayHello (Request1,Request2) returns (Response) {}//service has two inputs
  </code></pre>
  
  <p>//.proto file</p>
  
  <pre><code>syntax = "proto3";
  
  service Greeter{
      rpc SayHello (Request) returns (Response) {}
  }
  
  
  message Request{
      string request = 1;
  }
  
  message Response{
      string response = 1;
  }
  </code></pre>