add metadata/status info back into certificate.stable.k8s.psg.io objects

[stuart-warren]

It would be really nice to have some status information in the certificate.stable.k8s.psg.io objects so that users can debug issues or monitor their certificates, similar to pods having a status field.

I'm currently thinking fields like the expiry date which is otherwise only available in a secret and whether the certificate has actually been created yet.