Enhancement idea: Custom defines registers to be read

[madmat17]

A neat feature/enhancement to this plugin could be to also have the option to add custom registers to be read from the inverter. That could overcome some quite specific needs users are having e.g. with >2 strings (like reported here https://github.com/daolis/ioBroker.sun2000-modbus/issues/29#issue-2067493406), or other data points they would like to pull.