Provide callback method for API calls to avoid polling

Provide some callback method (webhook URL, push notification, etc) that an API call can use to indicate it's done. That way the user doesn't have to sit and poll endlessly. Some examples of where this would help: * After creating a restore job, waiting for the snapshot download URL to be ready. * After modifying the automationConfig, waiting for *all* agents to report that they're in goal state.