Add a user field

Reserving a slot for future use in application starters.

Change-Id: I04fda8d686f4b3af937550ee24f467c0cd40fdd6
diff --git a/aos/configuration.fbs b/aos/configuration.fbs
index 9a24c8a..890bad0 100644
--- a/aos/configuration.fbs
+++ b/aos/configuration.fbs
@@ -131,6 +131,10 @@
   // The node that this application will be started on.
   // TODO(austin): Teach starter how to use this for starting applications.
   node:string;
+
+  // The user to run this application as. If this field is unset, run it as
+  // the current user of the application starter.
+  user:string;
 }
 
 // Per node data and connection information.