Revisit Istio support

[yorugac]

Istio is supported in k6-operator but it'd be good to review it: its settings and general UX. See breaking change in #182 - honestly, this should have come up earlier.

In other words, Istio support in k6-operator could benefit from review and some testing. Consider implementing functional tests to have a way to quickly check backwards compatibility with Istio support. Maybe adding a guide doc on topic would be useful too.

The output here should be the following:

• a list of issues found while using Istio, esp. any difficulties in the setup or some functionality of k6-operator that is not working with Istio
• e2e tests with Istio setup (e.g. yaml files that can be applied with kubectl)
• a guide on how to setup k6-operator with the Istio setup