EXAMPLES: add basic event_hdl lua example script

For now: basic server event handling from lua, events are printed to
STDOUT.

The idea behind this is to provide simple and easy-to-use examples
that serve as a basic introduction for lua event handler feature.

examples/lua/README

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+These files are example lua scripts that can be customized
+if necessary.
+
+They can be loaded with the 'lua-load' keyword like this:
+
+    lua-load /path/to/lua_script.lua
+

examples/lua/event_handler.lua

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+-- haproxy event-handling from Lua
+--
+-- This file serves as a demo to show you the various events that
+-- can be handled directly from custom lua functions.
+-- Events captured from lua will be printed directly to STDOUT
+-- It may not be exhaustive, please refer to the lua documentation
+-- in doc/lua-api/index.rst for up-to-date content and further explanations
+
+-- subscribe to every SERVER family events, this is the equivalent of doing:
+-- core.event_sub({"SERVER_ADD", "SERVER_DEL", "SERVER_UP", "SERVER_DOWN"}, ...)
+core.event_sub({"SERVER"}, function(event, data)
+	-- This function will be called when:
+	--  - new server is added from the CLI (SERVER_ADD)
+	--  - existing server is removed from the CLI (SERVER_DEL)
+	--  - existing server state changes from UP to DOWN (SERVER_DOWN)
+	--  - existing server state changes from DOWN to UP (SERVER_UP)
+	-- If the server still exists at the time the function is called, data["reference"]
+	-- contains a valid reference to the lua server object related to the event
+	--
+        sv_status = data["reference"] ~= nil and data["reference"]:get_stats().status or "DELETED"
+        print("[DEBUG - FROM LUA]", "EventType." .. event .. ": " ..
+              "server " .. data["proxy_name"] .. "/" .. data["name"] .. " " ..
+              "is " .. sv_status)
+end)
+-- Please note that you may also use Server.event_sub() method to subscribe to events
+-- relative to a specific server only. See the lua documentation for more information.
+
+-- New event families will be added over time...