informers for multiple namespaces

currently the informers are only cluster wide or namespace specific.

I want to have an option like in controller-runtime to watch specific namespaces.

wide cluster access to secrets for example is problematic because of security concerns

I want it for the kafka-controller specifically but this is a general issue.

I already forked the kafka controller and override the default secret informer from this pacakge, so maybe this is something you would consider adding here.

what i basicly do is:

```
subs := make([]informers.SharedInformerFactory, 0, len(namespaces))
for _, ns := range namespaces {
subs = append(subs, informers.NewSharedInformerFactoryWithOptions(
client, resync, informers.WithNamespace(ns),
))
}
```
and then only for secrets create some wrap informer which maps the namespace to the dedicated namespaced informer

Start in the informers package at NewSharedInformerFactoryWithOptions and WithNamespace, then compare the per-namespace factories described in the issue with the existing cluster-wide and namespace-specific behavior. Define how a caller such as kafka-controller can watch selected namespaces, especially secrets, without cluster-wide access; done means the general package supports that option.

Written by the indexing model from the issue text.