Adds SoC Temperature + Batt Voltage + BootTime

[gargomoma]

With this additions we can track:

• Soc Temperature: It is not infalible, but can help to see if the board its heating, and also to counter frequency drift.
• Battery Voltage: Useful for solar stations. With this change the battery sensing pin and the voltage divider multplier could be set freely by the final user. Hopefully in the future we can merge it with the MQTT_Client::voltage() function. {"battery":{"pin":int XX, "scale": floatYY}} ie:{"battery":{"pin":15, "scale": 1.9}}
• BootTime: UnixTime in seconds. This could help to see if the board is having reboots.

I'm preparing more additions. You can see them here: https://github.com/G4lile0/tinyGS/compare/beta...gargomoma:tinyGS:beta_extras_246

[andypiper]

Battery voltage is super useful, thank you!