Feature/mqtt on main

[Cabooman]

What

Addition of MQTT subscribe/publish functionality, using the popular Pubsub library, with general data publishing and other examples

Why

Because we're all Home Assistant junkies whether we admit it (to ourselves) or not ❤️
On a more serious note, this allows the publication of data in a common format that can be stored/supervised/visualized or used for various home automation purposes. It also allows control of battery emulator functionality from various home automation frameworks if implemented.

How

• The PubSub library is added to the library stack.
• Settings for MQTT are added
• MQTT connection/initialization/loop/functions added

Global variables for data communication between inverter and battery are published every 5 s to the 'battery/info' topic (topic name can be changed to something better): SOC, SOH, MaxTemp, MinTemp, MaxCell, MinCell in JSON format, for example:

{
  "SOC": 68.500,
  "StateOfHealth": 92.000,
  "temperature_min": 5.600,
  "temperature_max": 7.200,
  "cell_max_voltage": 3812,
  "cell_min_voltage": 3782
}

Specific modules can publish whatever they want whenever they want to whatever topic they want. An example is included in
NISSAN-LEAF-BATTERY.cpp in the publish_data() function where 96 cell voltages are published every 5 seconds.

MQTT discovery for Home Assistant is available using the approach in NISSAN-LEAF-BATTERY.cpp. It's a bit more involved and is not necessary for Home Assistant integration, but it's really neat if done right