Validating webhook admission

[alex-bezek]

Description

The controller should install a Validating Admission Webhook that can do complex validations of created k8s resources have configurations that we can't support.

Kubebuilder Implementing Webhooks Guide - https://book.kubebuilder.io/cronjob-tutorial/webhook-implementation.html

Use Case

The store can't process some fields like default backends, rules without hosts, etc. Right now these are ignored and logged in the controller. It would be a much better user experience though if these validations were done by the k8s server when first applying and could then block bad config.

Related issues

https://github.com/ngrok/kubernetes-ingress-controller/issues/176