Add support for probe-returned key

After discussing what's needed to make ProbeBase work for other probing backends (IntelITT and NVTX in particular), it's clear that we need to support passing a backend-generated key from probe start to probe finish, at least when using a region/span API.

We could probably just assume that all backends will return a `UInt64` key from probe start, and accept it as an argument in probe finish just before user arguments are passed. To automate this, I think we should add an `@region` macro (name up for debate) that will generate the appropriate start/finish calls, and thread through the ID. I'm open to thoughts on what this should look like, especially w.r.t user-provided arguments (e.g. do we want to support passing arguments at probe finish, or at all?).

@simonbyrne @carstenbauer @maleadt