Initial skeletal implementation of the PrincipalService. Also, use the Login()/GetUserBlessing()
service from echo_client/echo_server.

R=jamesr@chromium.org, ataly@google.com

Review URL: https://codereview.chromium.org/1261403003 .
diff --git a/services/BUILD.gn b/services/BUILD.gn
index 13091a7..e6bd2b7 100644
--- a/services/BUILD.gn
+++ b/services/BUILD.gn
@@ -38,6 +38,9 @@
       "//services/sensors",
       "//services/speech_recognizer",
     ]
+    if (defined(go_build_tool) && go_build_tool != "") {
+      deps += [ "//services/vanadium/security:principal_service" ]
+    }
   }
 
   if (is_linux) {