Split out the non-Cohttp parts of prometheus-app

[ulrikstrid]

It would be useful to split out the parts that doesn't need cohttp to a separate library. That way we can build something that exposes it via Dream or Piaf whatever we want to use.

Related marigold-dev/deku#525

[ulrikstrid]

I created this repo which shows what I would want. The prometheus-app-pure is what I would like to have in this repo. If you think this makes sense I can create a PR to split it out.

[talex5]

That sounds sensible to me!