Define a setting and optional default on the extending class.

@param key [Symbol] @param default [Object]

@return [void]

A singleton settings container for storing immutable default configuration on the extending class.

@return [R10K::Settings::Container]

Allow subclasses to use the settings of the parent class as default values

@return [void]

A singleton settings container for storing manual setting configurations on the extending class.

@return [R10K::Settings::Container]