✨(backend) add admin Enrollment api endpoint

[jbpenrath]

Purpose

Add an admin api endpoint to manage Enrollment resource. Furthermore, an enrollment can be active/inactive. And finally, the enrollment list can filtered through the following criterias:

• query (course_run.title, course_run.resource_link, course_run.course.code)
• user_ids
• course_run_ids
• ids
• state
• is_active

Proposal

• [x] Allow to list and retrieve enrollments
• [x] Allow to active/inactive enrollments
• [x] Allow to filter enrollments