Support pre and post backup hooks

[NeurekaSoftware]

• pre backup hooks
• post backup hooks
• on failure hooks
• on warning hooks
• on success hooks

I have a push based monitor for Uptime Kuma and like sending a curl request on successful backup. Uptime Kuma sounds alarms if a successful backup isn’t sent as planned.

Being able to script with these hooks will allow us to handle any additional functionality that we need in zerobyte without muddying the codebase.