Add MQTT v5 support to MQTT event handler

conf/janus.eventhandler.mqttevh.jcfg.sample

@@ -16,6 +16,7 @@ general: {
 							# plain (no indentation) or compact (no indentation and no spaces)
 
 	url = "tcp://localhost:1883"	# The URL of the MQTT server. Only tcp supported at this time.
+	#mqtt_version = "3.1.1"			# Protocol version. Available values: 3.1, 3.1.1 (default), 5.
 	client_id = "janus.example.com"	# Janus client id. You have to configure a unique ID (default: guest).
 	#keep_alive_interval = 20		# Keep connection for N seconds (default: 30)
 	#cleansession = 0				# Clean session flag (default: off)
@@ -46,4 +47,7 @@ general: {
 	#tls_client_key = "/path/to/key.pem"
 	#tls_ciphers
 	#tls_version
+
+	# These options work with MQTT 5 only.
+	#add_user_properties = ()	# List of user property ["key", "value"] pairs to add.
 }