Principal Service: Add support for multiple user accounts

services/vanadium/security/BUILD.gn

 # Copyright 2015 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.
 
 import("//mojo/go/rules.gni")
 
 go_mojo_application("principal_service") {
   sources = [
+    "conventions.go",
+    "principal.go",
     "principal_service.go",
     "public_key.go",
     "wire_blessings.go",
   ]
   deps = [
     "//mojo/public/interfaces/network",
     "//mojo/services/authentication/interfaces",
     "//mojo/services/network/interfaces",
     "//mojo/services/vanadium/security/interfaces",
   ]
 }