Register for Google Classroom push notifications via Cloud Pub/Sub instead of polling

Steps

1. Create a Cloud Pub/Sub topic in your Developer Console project, then grant the service account classroom-notifications@system.gserviceaccount.com the pubsub topics.publish permission on that topic.
2. Call POST https://classroom.googleapis.com/v1/registrations (registrations.create) with a Registration body whose feed sets feedType to DOMAIN_ROSTER_CHANGES, COURSE_ROSTER_CHANGES, or COURSE_WORK_CHANGES (supplying courseId inside courseRosterChangesInfo or courseWorkChangesInfo for course-scoped feeds), plus cloudPubsubTopic.topicName pointing at your topic.
3. Authorize with scope classroom.push-notifications plus a matching data scope: classroom.rosters(.readonly) for roster feeds, or classroom.coursework.students(.readonly) for course work feeds.
4. Renew the subscription before it lapses by issuing an identical registrations.create call, since registrations expire after roughly a week and each call returns a fresh expiryTime and registrationId.
5. On each Pub/Sub message, read the collection field (e.g. courses.students, courses.courseWork.studentSubmissions) and resourceId map, which match the arguments of that resource's get method; use the registrationId message attribute with registrations.delete to unsubscribe.