feat(alerting): Implement ntfy provider

Closes #308

Work remaining:
- Add the documentation on the README.md
- Test it with an actual Ntfy instance (I've only used https://ntfy.sh/docs/examples/#gatus as a reference; I haven't actually tested it yet)

config/config.go

@@ -306,6 +306,7 @@ func validateAlertingConfig(alertingConfig *alerting.Config, endpoints []*core.E
 		alert.TypeMatrix,
 		alert.TypeMattermost,
 		alert.TypeMessagebird,
+		alert.TypeNtfy,
 		alert.TypeOpsgenie,
 		alert.TypePagerDuty,
 		alert.TypeSlack,