webhook/callback on status change

[aidantrabs]

summary

notify external systems when a target's status changes (e.g. healthy → unhealthy).

details

• detect status transitions (not just current status)
• support a callback interface, e.g. OnStatusChange(target string, from, to Status)
• provide a built-in webhook option that POSTs json to a configured url
• consider a functional option like WithOnStatusChange(fn) to keep the api consistent