Webhook namespace selector based on labels

### What feature you would like to be added?

`spark-operator` Controller can filter watched Namespaces either by their name or by label selectors. While this is very convenient for shared clusters, unfortunately this is not the case for the webhook.

The webhook can filter Namespace only based on their name. The command-line argument is there but it is never used. It would complement perfectly if webhook was filtering Namespaces based on the same labels as the controller.

### Why is this needed?

This is needed in a shared Kubernetes cluster where multiple `spark-operator` instances are running simultaneously and the namespaces are not known in advance.

### Describe the solution you would like

I suppose something similar to the existing implementation for the controller https://github.com/kubeflow/spark-operator/issues/2479

### Describe alternatives you have considered

_No response_

### Additional context

_No response_

### Love this feature?

Give it a 👍 We prioritize the features with most 👍

Locate the webhook's namespace filtering and the existing command-line argument, then read the controller's label-selector implementation referenced in issue #2479. Verify how the webhook currently filters by namespace name, add matching label-based behavior, and test that only namespaces selected by the configured labels are watched.

Written by the indexing model from the issue text.