Add support to write setpoint values received from MQTT into configurable registers

[didier-wenzek]

I'm looking for something similar to what is done to read measurement values from Modbus registers:

• Listen for setpoint values published over MQTT
• Use a configuration file to associate registers to the setpoint names used over MQTT
• Write the configured registers with the values received from MQTT.

An open question, to be addressed in sync with thin-edge, is on what topic should the setpoint values be published?

1. An option is to use a convention aligned with measurements: te/+/+/+/+/sp/ where sp stands for setpoint.
2. An alternative is to use the same convention as for commands: te/+/+/+/+/cmd/setpoint/

[zhongys-c8y]

I give my vote to the option2. It should be a kind of operation, not measurement.

[rina23q]

I also vote for using cmd. The requirement is only to write Holding Registers? Not writing Coils?

[zhongys-c8y]

The requirement is only to write Holding Registers? Not writing Coils?

I would say both

[didier-wenzek]

Implemented by #39

[reubenmiller]

Included in the release v0.10.0