MQTT: Add Online / Last Will messages (LWT)

[aly-fly]

As you are not accepting PRs, I am sending the recommended updates here.
Changes below implement Online and Offline status, which can be used by high-level MQTT systems, like HA, to correctly handle the disconnected device.

List of global variables - add:

String               mqttalivemsg;                       // MQTT mesasage for online status
String               mqttlwtmsg;                         // MQTT mesasage for LWT (Last Will and Testament)

List of definistions - add:
#define MQTT_CONN_SUBTOPIC "connection" // Subtopic where "online", "offline" and Last Will messages are published

M Q T T P U B _ C L A S S -
enum add:
MQTT_ALIVE

amqttpub[10] add
{ MQTT_CONN_SUBTOPIC, MQSTRING, &mqttalivemsg, false }, // Definition for MQTT_ALIVE
and increase number of items from 9 to 10.

M Q T T R E C O N N E C T:
mqttclient.connect replace with:

mqttlwtmsg = "offline"; 
  sprintf ( subtopic, "%s/%s",
            ini_block.mqttprefix.c_str(),
            MQTT_CONN_SUBTOPIC ) ;

  res = mqttclient.connect ( clientid,                    // Connect to broker and set LWT message at the broker
                             ini_block.mqttuser.c_str(),
                             ini_block.mqttpasswd.c_str(),
                             subtopic,
                             0,
                             false,
                             mqttlwtmsg.c_str() ) ;

Documentation:
https://www.hivemq.com/blog/mqtt-essentials-part-9-last-will-and-testament/

Result:

When network connection breaks for any reason, the MQTT broker publishes the connection = offline message.