beaconobject: Created a ConfigProperty

Feature a ConfigProperty to replace config_getter.

This allow to describe a simple property from the configuration, which don't have to be stored locally as a setting. It was previously featured by config_getter.

This allow to use obj.apply_config(True), obj.config.reload(), obj.config.save(), so it is properly synchronized (like config_getter).

It can be used together with a BeaconObject (which is tested) or a ConfigItemContainer (which is actually not tested).

This feature was designed in a reduced version at bliss-tomo, i would like to replace it with a stronger concept from BLISS (tomo/bliss-tomo!477 (merged)).

• config_getter is limited and part of BeaconObject, so it is also exposed in the final object in the beamline, which is not a good idea.
• ConfigProperty provides the same thing + type checking, read only, mandatory/default value