Account for external DNS operational in cluster availability

https://redhat-internal.slack.com/archives/G01QS0P2F6W/p1673871354403469

Quoting Cesar:

"We can have canary checks in the same way the router does. However, we need to assume that the external endpoint for hosted clusters is accessible from the hypershift operator. That may be the case for ROSA, but not always, so we would have to make it optional."

I think we could also infer this from the status of some of the Services or resources that are exposed to the external DNS component.