MINOR: hlua/event_hdl: expose proxy_uuid variable in server events

Adding proxy_uuid to ServerEvent class.
proxy_uuid contains the uuid of the proxy to which the server belongs
diff --git a/doc/lua-api/index.rst b/doc/lua-api/index.rst
index f25f9dc..1bb4997 100644
--- a/doc/lua-api/index.rst
+++ b/doc/lua-api/index.rst
@@ -1405,6 +1405,10 @@
 
   Contains the name of the proxy to which the server belongs
 
+.. js:attribute:: ServerEvent.proxy_uuid
+
+  Contains the uuid of the proxy to which the server belongs
+
 .. js:attribute:: ServerEvent.reference
 
   Reference to the live server (A :ref:`server_class`).