Add JSON settings definition format and conversion to C code

[TjazVracko]

Context

a JSON file defines each setting (id, key, type, default value).
Some Kconfig option enables JSON->C code conversion (probably one with the relative path to JSON and one to enable the conversion).
The code will read the JSON and generate applicable user_settings_add functions, so they can be used from the firmware.

Required steps / Implementation details

The following step are required:

• Python script for code generation from JSON file (JSON -> user settings) #21
• configuring and calling the script via Kconfig and CMake. #22

Check how this is done in b-braun.
The JSON should be as similar as possible to simplify the use for our phone app developpers.

Definition of Done

• JSON format is defined
• conversion code exists and is tested
• Kconfig options exist to enable this functionality in a project
• code is reviewed

[eva-cerncic]

It would be useful to add "get default value", so we can also auto-generate function that will set default values from json structure if the new default value is different from the set one or default is not set yet.