feat: implement catalog course enrollment service

[Ang-m4]

Description

This pull request refactors the enrollment logic to align with the current business requirements.
It adds a new enroll endpoint inside the learner API so that course enrollments can be triggered directly from the learning paths MFE.

Key changes

• Added a new enroll endpoint at /catalogs/<catalog_id>/courses/<course_id>/enroll, called as the learner user, to create the CatalogCourseEnrollment.
• Introduced a new CatalogCourseEnrollmentService to centralize all course enrollment operations (create, activate, deactivate) with consistent validation.
• Added an enrollments.py policies module to enforce business rules and keep validation logic separate from the service layer.
• Ensured that, when this endpoint is called, the corresponding edX enrollment is also created (or updated) in the platform as needed.