MQTT Datasource Work

[hobobandy]

• Added parsing the JSON payload to fill required metadata for datasources to parse packets properly (wish there was a way to prevent JSON parsing twice, but it probably requires major refact in the datasources?)
• Added a helper function to reduce repeating code
• Changed comparisons to use .empty()

Tested with MQTT messages from readsb, Network Survey App WiFi and Bluetooth transformed into expected JSON using Node-RED. I haven't tested against rtl_433 since there aren't sensors around me, but any JSON payload formatted the way a Kismet datasource expects it to be should be supported.

Example configuration/Node-RED flows: https://hobobandy.tips/guides/mqtt/kismet-mqtt/