def optional_target_names(target_type, key = nil)
      optional_targets(target_type, key).map { |optional_target| optional_target.to_optional_target_name }
    end
def optional_target_names(target_type, key = nil)
      optional_targets(target_type, key).map { |optional_target| optional_target.to_optional_target_name }
    end
Returns optional_target names of the notification from configured field or overriden method.
 This method is able to be overriden.

 @param [String] target_type Target type to notify
 @param [String] key Key of the notification
 @return [Array<Symbol>] Array of optional target names