fix: enrollments serializer for multiple courses

[shadinaif]

We've thought of this the wrong way 😒

CourseScoreAndCertificateSerializer contains the optional field exam_scores; which requires running collect_grading_info before rendering it. LearnerDetailsForCourseSerializer is doing that cleanly here.

We changed the logic of the collector to support multiple courses here. And that's a mistake 🤕 ! The report should not include more than one course if exam_scores is requested. Each course has its own set of columns; having all columns of all selected courses in the header is a very bad idea. Sorry, it just didn't cross my mind before now!

What's the goal we want to achieve, from the user perspective?

• if the enrollment API is called with course_ids containing one-and-only-one course; then the response can include exam scores columns when using optional_fields_tags=csv_export
• if the enrollment API is called with course_ids containing more than one course, or called without course_ids; then the response will not include exam scores columns even if optional_fields_tags=csv_export is used

How to do that in the code?

• Revert the support for multiple courses in collect_grading_info. It should support only one course
• Check for course_ids in LearnerEnrollmentSerializer. If it does not contain one-and-only-one course-id; then remove exam_scores from requested_optional_field_tags of the serializer's context (if requested). See how this method checks for requested tags