Changes to GET `/api/v0/main-frontend/course-auditing`

added the required property items/is_draft to the response with the 200 status

added the required property items/is_joinable_by_code_only to the response with the 200 status

added the required property items/is_test_mode to the response with the 200 status

added the required property items/is_unlisted to the response with the 200 status

endpoint added