Custom Domain and TLS monitoring

In press, we are facing this issue that sometimes user changed their custom domain's DNS or the domain itself gets expired. Press keeps assuming that the site is down, where from press-end, the issue can't be resolved.

In pilot,
- it can monitor the custom domain's reachability
- check whether cloudflare or anything is blocking the ping endpoint, better to have some fallback of it (any idea ?)
- check whether user changed the dns and it's not pointing to it anymore. In that case, notify user.
- query whois rdap (https://www.iana.org/) for expiry date of domain
- if domain is expired, pilot can send notification to users also beforehand and can stop monitoring of it.

No files, tests, or entry points are named. Begin by tracing how Pilot currently decides a site is down, then define completion criteria for reachability fallback, DNS changes, RDAP expiry notifications, and stopping monitoring; confirm the behavior with maintainers before implementation.

Written by the indexing model from the issue text.