add signup method

api/main.proto

@@ -17,6 +17,12 @@ service EveningDetectiveServer {
       post: "/api/echo"
     };
   }
+
+  rpc Signup(SignupReq) returns (SignupRsp) {
+    option (google.api.http) = {
+      post: "/api/signup"
+    };
+  }
 }
 
 message PingReq {}
@@ -30,3 +36,12 @@ message EchoReq {
 message EchoRsp {
   string text = 1;
 }
+
+message SignupReq {
+  string name = 1;
+  string email = 2;
+}
+
+message SignupRsp {
+  string error = 1;
+}