Added Entitlement enroll and unenroll logic to the Enrollment API

[LEARNER-3136]

This commit containts the logic and API endpoint for a user to Enroll,
Unenroll, and switch-session on a Course Entitlement.

openedx/core/djangoapps/catalog/utils.py

@@ -235,7 +235,6 @@ def get_course_runs_for_course(course_uuid):
             cache_key=cache_key if catalog_integration.is_cache_enabled else None,
             long_term_cache=True
         )
-
         return data.get('course_runs', [])
     else:
         return []