Support --source=gateway

[rupurt]

How would you feel about publishing records for the gateway source in addition to the current service & ingress?

For example with the envoy gateway the published record has an auto-generated name which makes it annoying to route & remember. It would be awesome if we could publish a logical name based on the gateway resource definition e.g.

NAMESPACE        NAME   CLASS   ADDRESS      PROGRAMMED   AGE
my-namespace      eg     eg      172.18.0.2   True         4m27s

> getent hosts eg.my-namespace.local

[blake]

@rupurt I think that'd be a great addition.