Operator namespace restriction proposal

[Keith W]

https://github.com/kroxylicious/design/pull/91

TL;DR:

Proposal:

Allow the Kroxylicious Operator to be restricted to watching a static, user-defined set of
Kubernetes namespaces via an environment variable, rather than always watching the entire cluster.

Motivation:

On a large shared Kubernetes or OpenShift cluster there can be hundreds or thousands of `Secret` and `ConfigMap` objects unrelated to Kroxylicious. Because the operator caches all of them, there is the possibility that the Operator may run out of memory.

A more targeted solution is to allow operators to restrict the operator to the namespace(s) in which Kroxylicious resources will actually be deployed.