Add a monitor type for upstream status pages

[thislooksfun]

📑 I have found these related issues/pull requests

I think #6028 is a strict subset of this feature request -- specifically for adding an "upstream" status page where the host is Uptime Kuma.

🏷️ Feature Request Type

New monitor

🔖 Feature description

In the modern world it is very common for services to rely on upstream services. It would be really nice if Uptime Kuma had a way cleanly add the status of upstream services to a status page. For example, if you rely on Twilio for sending text messages you could link the Twilio Status page, or if you take payments you could have an entry for Stripe.

✔️ Solution

I would like a way to add the overall status of an upstream status page to my Uptime Kuma status page, without needing to build a bespoke HTTP monitor every time. Since most places I've encountered use a relatively small set of status page tooling (Atlassian Statuspage and Instatus are by far the most common I've personally seen), having a first-class integration for those would make tracking the status of third-party services very user-friendly.

Ideally this would be as simple as:

1. Make a new monitor
2. Set the type to "status page"
3. Enter a URL
4. Select the underlying technology (Atlassian Statuspage / Instatus / Uptime Kuma / etc). Ideally this will be able to detect the platform automatically, but the user should be able to override it if it gets it wrong somehow

❓ Alternatives

We could build custom HTTP monitors for each upstream status page we want to track, but that gets complicated quickly, especially since I would ideally like to see more fine-grained status (e.g. is the upstream fully down? or is it just degraded?).

📝 Additional Context

No response