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/go/rules.gni")
+
+group("go") {
+  assert(go_build_tool != "")
+  deps = [
+    ":go_principal_service",
+  ]
+}
+
+go_mojo_application("go_principal_service") {
+  sources = [
+    "principal_service.go",
+  ]
+  deps = [
+    "//mojo/services/vanadium/security/public/interfaces",
+  ]
+}