Initial skeletal implementation of the PrincipalService. Also, use the Login()/GetUserBlessing()

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/public/mojo_application.gni")
+import("//mojo/go/rules.gni")
 
-mojo_native_application("tiny") {
+go_mojo_application("principal_service") {
   sources = [
-    "tiny.c",
+    "principal_service.go",
+  ]
+  deps = [
+    "//mojo/services/vanadium/security/public/interfaces",
   ]
 }