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

services/BUILD.gn

Index: services/BUILD.gn
diff --git a/services/BUILD.gn b/services/BUILD.gn
index 13091a7585973142407ccb8532fe71cfa8c43e52..27e1b1a5d9886cc989ba0576c1dc5c938681df8b 100644
--- a/services/BUILD.gn
+++ b/services/BUILD.gn
@@ -40,6 +40,10 @@ group("services") {
     ]
   }
 
+  if (defined(go_build_tool) && go_build_tool != "") {
+    deps += [ "//services/vanadium/security:go_principal_service" ]
+  }
+
   if (is_linux) {
     deps += [ "//services/python" ]
   }