MINOR: hlua: expose proxy mailers

Proxy mailers, which are configured using "email-alert" directives
in proxy sections from the configuration, are now being exposed
directly in lua thanks to the proxy:get_mailers() method which
returns a class containing the various mailers settings if email
alerts are configured for the given proxy (else nil is returned).

Both the class and the proxy method were marked as LEGACY since this
feature relies on mailers configuration, and the goal here is to provide
the last missing bits of information to lua scripts in order to make
them capable of sending email alerts instead of relying on the soon-to-
be deprecated mailers implementation based on checks (see src/mailers.c)

Lua documentation was updated accordingly.

doc/lua-api/index.rst

@@ -1021,6 +1021,8 @@ Core class
 
   Use this when sending email alerts directly from lua.
 
+  :see: :js:func:`Proxy.get_mailers()`
+
 .. _proxy_class:
 
 Proxy class
@@ -1143,6 +1145,63 @@ Proxy class
    proxy.
   :returns: a key/value table containing stats
 
+.. js:function:: Proxy.get_mailers(px)
+
+  **LEGACY**
+
+  Returns a table containing mailers config for the current proxy or nil
+  if mailers are not available for the proxy.
+
+  :param class_proxy px: A :ref:`proxy_class` which indicates the manipulated
+   proxy.
+  :returns: a :ref:`proxy_mailers_class` containing proxy mailers config
+
+.. _proxy_mailers_class:
+
+ProxyMailers class
+==================
+
+**LEGACY**
+
+.. js:class:: ProxyMailers
+
+  This class provides mailers config for a given proxy.
+
+  If sending emails directly from lua, please consider
+  :js:func:`core.disable_legacy_mailers()` to disable the email sending from
+  haproxy. (Or email alerts will be sent twice...)
+
+.. js:attribute:: ProxyMailers.track_server_health
+
+  Boolean set to true if the option "log-health-checks" is configured on
+  the proxy, meaning that all server checks event should trigger email alerts.
+
+.. js:attribute:: ProxyMailers.log_level
+
+  An integer, the maximum log level that triggers email alerts. It is a number
+  between 0 and 7 as defined by option "email-alert level".
+
+.. js:attribute:: ProxyMailers.mailservers
+
+  An array containing the list of mail servers that should receive email alerts.
+  Each array entry is a name:desc pair where desc represents the full server
+  address (including port) as described in haproxy's configuration file.
+
+.. js:attribute:: ProxyMailers.smtp_hostname
+
+  A string containing the hostname to use for the SMTP transaction.
+  (option "email-alert myhostname")
+
+.. js:attribute:: ProxyMailers.smtp_from
+
+  A string containing the "MAIL FROM" address to use for the SMTP transaction.
+  (option "email-alert from")
+
+.. js:attribute:: ProxyMailers.smtp_to
+
+  A string containing the "RCPT TO" address to use for the SMTP transaction.
+  (option "email-alert to")
+
 .. _server_class:
 
 Server class