Allow use of IP_ADDRESS.nip.io addresses as the hostname field of a Gateway

[ca-scribner]

Enhancement Proposal

Continuing the discussion from this comment, we should allow hostname to be set to the external IP or IP.nip.io of the loadbalancer service. The Gateway docs say IP address is not valid, but it does work (the controllers dont reject it) and the regex definition supports it (try testing it here)

Where this is an issue is that Kubeflow uses .nip.io urls to connect to a local install of kubeflow as if you're routing to a public host (search nip.io here). Presently, our regex blocks the use of this type of url.

There might also be merit in allowing IPADDRESS (without nip.io) as a host here, as it does work even if its not stated as supported.