Improve UX for overriding Proxy Image Registry

> this is supported using the `envoyProxy` field in the EnvoyGateway config https://gateway.envoyproxy.io/docs/api/extension_types/#envoygateway

This is maybe technically true, but I don't think that's a good UX.

The issue is that when I set global image config (either or both of `.global.imageRegistry` and `.global.imagePullSecrets`), that config does not properly propagate fully to the proxy pod. Worse, the shutdown container _will_ be affected by `.global.imageRegistry`, but not the proxy container, and the pod won't receive the imagePullSecrets set in `.global.imagePullSecrets`. Therefore, it will not start. This is bad!

It seems like a bad UX to allow setting `.global.imageRegistry` and have it partially applied to the proxy pod, requiring the gaps to be filled in with config like (as you suggest):

```yaml
config:
envoyGateway:
envoyProxy:
provider:
kubernetes:
envoyDeployment: # or envoyDaemonSet
pod:
imagePullSecrets:
- name: wibble
```

...which still doesn't configure the envoy proxy container image, so you'd need to use `patch` on the `KubernetesDeploymentSpec` for that. And now you have to specify the whole image rather than just the registry defined in `.global.imageRegistry`.

_Originally posted by @krishicks in https://github.com/envoyproxy/gateway/issues/8307#issuecomment-3941464930_