feature: Add option to disable unauthorizedUserAccessSpec for vmauth in victoria-metrics-k8s-stack helm chart

[mfcmaestro]

Chart name and version, where you feel a lack of requested feature
chart: victoria-metrics-k8s-stack
version: v0.35.7

Is your feature request related to a problem? Please describe.
In current implementation unauthorizedUserAccessSpec is enabled by default in vmauth template, so there is no option to disable it in Values.

Describe the solution you'd like
It would be nice to have an option to disable (for example set it to false) unauthorizedUserAccessSpec in vmauth to deny all unauthorized access and manage it only via users.

[ctrlsam]

This also applies closely to the cluster chart.

The VictoriaMetrics Helm chart automatically appends an unauthorized_user block whenever vmauth is enabled, so even if you provide your own users: [...] configuration, you still end up with a unauthorized user entry in the final auth.yml. Currently, the chart’s logic doesn’t provide a direct toggle for removing the unauthorized_user block.

https://github.com/VictoriaMetrics/helm-charts/blob/master/charts/victoria-metrics-cluster/templates/vmauth-config.yaml