This introduces a `Hooks` service. It registers hooks from config and
dispatches hook events at runtime.
N.B. The hook config is not wired up to this yet. But for legacy
reasons, we wire up `notify` from config and power it using hooks now.
Nothing about the `notify` interface has changed.
I'd start by reviewing `hooks/types.rs`
Some things to note:
- hook names subject to change
- no hook result yet
- stopping semantics yet to be introduced
- additional hooks yet to be introduced
gt-oai