Sync a Google Classroom course roster by listing and adding students and teachers

Steps

1. Call GET .../courses/{courseId}/students (courses.students.list) and GET .../courses/{courseId}/teachers (courses.teachers.list) to read the current roster; requires scope classroom.rosters or classroom.rosters.readonly (add classroom.profile.emails/classroom.profile.photos for those profile fields).
2. Call POST .../courses/{courseId}/students (courses.students.create) to add a student directly — permitted for domain administrators adding within-domain users; a non-admin adding themselves must supply the course's enrollmentCode as a query parameter.
3. Call POST .../courses/{courseId}/teachers (courses.teachers.create) to add a teacher directly, likewise restricted to domain administrators adding within-domain users.
4. For any caller that isn't a qualifying domain admin — including teachers adding other teachers or students — use invitations.create instead so the invitee can accept.
5. Handle ALREADY_EXISTS when a user is already a student/teacher, and FAILED_PRECONDITION errors such as CourseMemberLimitReached or CourseTeacherLimitReached during sync.