[MQTT] Add properties to the mqtt event

[TomerShor]

The MQTT topic was previously populated in the URL field of the event, and was confused with the recently added (empty) topic property.

In this PR I added some fields to the MQTT event, as suggested in #3170 :

• URL: The URL of the MQTT broker
• topic: The topic of the message
• path: The topic of the message (alias for topic)
• body: The message payload
• id: The message id

Resolves #3170

[woodzy]

Thanks -- expanding the internal fields available to MQTT is appreciated.