litrpc: add revoked_at field to Session proto

Author: ellemouton

litrpc/lit-sessions.pb.go

@@ -92,6 +92,16 @@ message Session {
     MacaroonRecipe macaroon_recipe = 12;
 
     string account_id = 13;
+
+    /*
+    The unix timestamp indicating the time at which the session was revoked.
+    Note that this field has not been around since the beginning and so it
+    could be the case that a session has been revoked but that this field
+    will not have been set for that session. Therefore, it is suggested that
+    readers should not assume that if this field is zero that the session is
+    not revoked. Readers should instead first check the session_state field.
+    */
+    uint64 revoked_at = 14 [jstype = JS_STRING];
 }
 
 message MacaroonRecipe {