Allow IP to be specified for Apex domains

[maleck13]

What

If someone adds an apex domain as a listener in the gateway, we will fail to setup DNS for that as apex domains can only have A records.

Solution

Allow the user to specify an IP for any apex domains in the DNSPolicy which we will use as the A record value. The user can then for example setup a 301 redirect to re-route traffic to a subdomain where DNS Load Balancing has been applied

This is a change the DNSPolicy and so will need a short RFC:

spec:
  apexAddress: 82.17.241.23 #any apex covered by the DNSPolicy gets this IP and then a redirect to www. etc can be applied

[philbrookes]

potential alternative: apexRedirect

Allow a customer to define a hostname to redirect to from the apex domain, the kuadrant-operator could then configure a listener in the gateway to perform this redirect and instruct (via DNSRecord) the dns-operator to use the IPs of the gateway (resolving CNAMEs) for the apex domain.

ping @maleck13