Task - RBAC AuthZ - Implement feature flag for AuthZ for course authoring

[rodmgwgu]

Define Waffle flag for enabling AuthZ for course authoring.

Only implement flag definition for using on endpoint validations in the future.

Flag name: authz.enable_course_authoring

See: https://docs.openedx.org/projects/edx-toggles/en/latest/how_to/implement_the_right_toggle_type.html#implementing-the-right-toggle-class for best practices