Provide Network Policy Generation Environment Variable

Provide the possibility to completely disable network policy generation. This is useful in environments with strict and evolving security requirements, which are enforced by using the admission controller Kubernetes feature. In that case, the user would be able to define its own custom network policies.

This feature is already proposed upstream here:

https://github.com/strimzi/proposals/blob/main/028-network-policy-generation-environment-variable.md

With a companion pull request here:

https://github.com/strimzi/strimzi-kafka-operator/pull/5258