Binary protocol ACK responses to SET messages

[TjazVracko]

Context

Currently, a SET message has no built-in response. The user must issue a GET command to see if the SET was successful.
This is not good for non-realtime uses of the binary protocol (like LoRaWAN). It would be nice to schedule a SET on the server and receive a GET of the same setting back, or some kind of ACK/NACK

Required steps / Implementation details

Two options:

• implement an ACK/NACK response for SET commands
• implement add GET response to SET and GET_FULL response to SET_DEFAULT. What happens if the SET fails in this case?

We must also test the robustness of the protocol when "random" values are sent, when a valid setting is set with a wrong value len, ... (We need unit tests for the user_settings_protocol_executor, basically).

Definition of Done

SET command generates some kind of response.