Add Knative support to dex chart

[kahirokunn]

Overview

Add optional Knative Service support to the dex Helm chart.

What this PR does / why we need it

• Adds knative.enabled to render a Knative Service instead of the default Kubernetes Deployment and Service.
• Supports Knative revision settings such as annotations, container concurrency, timeouts, and min/max scale.
• Enables Knative-managed routing and autoscaling, including scale-to-zero for idle development or off-hours environments.
• Lets Knative buffer requests during sudden traffic spikes, reducing direct pressure on dex while new replicas are started.
• Updates install notes and documentation for Knative mode.
• Bumps the chart version to 0.25.0 and adds an Artifact Hub changelog entry.

Special notes for your reviewer

When knative.enabled=true, Kubernetes ingress, httpRoute, autoscaling, and podDisruptionBudget should remain disabled because Knative manages routing, request buffering, and autoscaling for the workload.

Checklist

• Change log updated in Chart.yaml (see the contributing guide for details)
• Chart version bumped in Chart.yaml (see the contributing guide for details)
• Documentation regenerated by running make docs