Utility methods for reading configSet properties.
One purpose of this notion is to express immutability.
The contents are not used within the config itself; do not confuse this with config user-defined properties.
The properties are stored as a JSON file within the configSet that we read into a NamedList.
It's optional; there is no file if there are no properties.
Note that this logic is also used to load configSet flags; see ConfigSetService.