[layers v2] Decide project-wide hooks vs. layer-specific hooks

Part of #9765.

Decide on and implement support for layer-specific hooks.

This is just extending the abstraction of project-level hooks, so that layers can *also* have their own scoped hooks. There was some discussion about this, and @tg-msft + @vhvb1989 can provide some more details here.

- Today, layer hooks only support `preprovision`/`postprovision` — a holdover from layers being infra-only. Project hooks cover the full lifecycle, service hooks cover build/package/publish/deploy/restore.
- Without this, micro-templates won't have a way to have hooks apply *only* to their layer — project level is too broad.