Added getters for IP, Domain, and Port.

[AdamJHowell]

I wanted a way to query the PubSubClient object, for cases where I attempt to connect to multiple different brokers with one PubSubClient object, and for cases where I have multiple PubSubClient objects which are passed around to helper functions.

This has been tested on an ESP32 WROOM with the following block:

if( mqttClient.connected() )
{
	Serial.print( "MQTT broker domain: " );
	Serial.println( mqttClient.getServerDomain() );
	Serial.print( "MQTT broker IP: " );
	Serial.println( mqttClient.getServerIP() );
	Serial.print( "MQTT broker port: " );
	Serial.println( mqttClient.getServerPort() );
}