Support for nested MQTT messages

[The-JC]

Added support for more complex(nested) MQTT messages, as some devices publish nested contents (for example Tasmota). To handle this, the metrics are recursively get parsed, to extract all metrics from a message.

An example would be this MQTT message:

{
    "Time": "2021-10-03T11:05:21",
    "ENERGY": {
        "Total": 152.657,
        "Yesterday": 1.758,
        "Today": 0.178,
        "Power": 143,
        "ApparentPower": 184,
        "ReactivePower": 117,
        "Factor": 0.77,
        "Voltage": 235,
        "Current": 0.784
    }
}

It will parse into:

mqtt_ENERGY_Total
mqtt_ENERGY_Yesterday
mqtt_ENERGY_Today
mqtt_ENERGY_Power
mqtt_ENERGY_ApparentPower
mqtt_ENERGY_ReactivePower
mqtt_ENERGY_Factor
mqtt_ENERGY_Voltage
mqtt_ENERGY_Current

[kpetremann]

Thanks for the PR :)