Implement new Sungrow encrypted modbus comms

[michael-robbins]

Sungrow recently pushed out a firmware update to their Wifi dongles that use an encrypted modbus comms method.

A user on Whirlpool has successfully figured out how the encryption works, we'd just need to subclass ModbusSocketFramer to implement the encryption/decryption and discovery of the inverter key (assuming it's different per inverter).

Whirlpool link: https://whrl.pl/Rf8BQT

[michael-robbins]

There is a working implementation about to be merged into https://github.com/meltaxa/solariot/tree/%2312-Use-SungrowModbusTcpClient

Can probably be ported over to here easily enough!