Health status change hooks

[natalie-zamani]

We should provide a way to hook into health status changes, to allow triggering of certain actions on status changes.

For instance, if an app is marked unhealthy, then one might want to trigger some sort of action, like an alert, and email, or whatever.

Whether this should be only at the application health level, or at the individual dependency health level as well, is somewhat up for discussion in my mind.