Consider having a built-in adapter for Prometheus

While Cachet currently only concerns itself with displaying statuses (and maintenance messages) for components, these are still to be created/updated manually via the Dashboard (or API). Of course there is the possibility of integrating with Oh Dear, but in the interest of presenting a truly open-source solution that kinda 'closes the loop' it'd be great to have a built-in integration with Prometheus.

At least in the past with v2, there was reasonable demand for such a solution, as elicited by the following 3rd party adapters (which unfortunately are not maintained anymore, and probably don't play well with v3):

- https://github.com/gregdhill/prometheus-cachet
- https://github.com/oxyno-zeta/prometheus-cachethq

As far as I can see, this mostly revolves around having a translator for the [webhook events from Prometheus Alertmanager](https://prometheus.io/docs/alerting/latest/configuration/#webhook_config), some way to map the individual entities to components, and of course resolving incidents when they stop firing.

Looking forward to further discussions on this matter!