MINOR: hlua_fcn: alternative to old proxy and server attributes

MINOR: hlua_fcn: alternative to old proxy and server attributes

This patch adds new lua methods:
- "Proxy.get_uuid()"
- "Proxy.get_name()"
- "Server.get_puid()"
- "Server.get_name()"

These methods will be equivalent to their old analog Proxy.{uuid,name}
and Server.{puid,name} attributes, but this will be the new preferred
way to fetch such infos as it duplicates memory only when necessary and
thus reduce the overall lua Server/Proxy objects memory footprint.

Legacy attributes (now superseded by the explicit getters) are expected
to be removed some day.

Co-authored-by: Aurelien DARRAGON <adarragon@haproxy.com>