Add duo_features_enabled cascading setting

* `duo_features_enabled` was already an attribute on the
  `project_settings` table
* This MR uses the cascading settings framework to add this attribute to
  the `namespace_settings` table as well: https://docs.gitlab.com/ee/development/cascading_settings.html
* Other cascading settings assume that projects inherit the setting
  value from their parent group. For this setting, we want each project
  to be able to have a distinct setting value that may be different from
  its parent group. To do this, a new `CascadingProjectSettingAttribute`
  module was added that has very similar functionality to the
  `CascadingNamespaceSettingsAttribute` module.
* THis MR adds the cascading setting and behavior but the setting will
  not affect Duo Chat or Code Suggestions settings until a follow-on MR.
* https://gitlab.com/gitlab-org/gitlab/-/issues/441481

Changelog: added
EE: true