Add status field to API

We need a way to communicate to the user that the CR is misconfigured. Also, whether or not a Gateway exists in order to implement Policy Attachment correctly (see [here](https://github.com/istio/api/pull/2888#issuecomment-1673384025)).

**Task:**
The status of the targetRef referenced Gateway needs to be exposed to the controller responsible for applying policy. (One idea is to add a status field to Istio/api, but we should also consider existing solutions that may exist (e.g. kstatus)).