In #4021, @BrandonY asked for convenience wrapprer(s) for the following:

• Create the user-supplied topic
• Set the appropriate IAM policy needed to support bucket notifications
• Create a subscription to the topic

Adding this support would make storage depend on pubsub (at least optionally), and would complicate our testing story (system tests already add it). Ruby just documents what is required.

@danoscarmike, @lukesneeringer nominated you as the arbitrator for the issue.