Add a custom setting feature implementation

[nbatterham]

A common feature check could be to use a custom setting which can then in turn be enabled per user/per profile or globally

One consideration is that for this to work - the feature setting will also need to define the api name of the custom setting being used as we cannot use a default like we did for CustomMetadataEnabledFeatures

One possible implementation will be to add a new field to Feature Setting Custom Metadata that must be populated if the implementation is the CustomSettingEnabledFeature. On top of this we will need to be able to pass this value once the FeatureSetting is instantiated - we could potentially use aMap<String, String> options parameter and add an interface method to set the options on all Feature implementations