knative: implement dynamic sidebar filtering based on CRD discovery

[Joshna907]

Summary

This PR refactors the Knative plugin's menu registration in the sidebar to implement dynamic, cluster-aware visibility filtering. It hides the entire Knative menu group when connected to a cluster where Knative's Serving components are not installed, matching Headlamp's visual quality standards.

Key changes

• knative/src/index.tsx:
  • Imported K8s, Utils, and registerSidebarEntryFilter from the plugin SDK.
  • Implemented checkKnativeInstalled(cluster) with a lightweight caching and time-to-live (TTL) mechanism to search the cluster's Custom Resource Definitions (CRDs) for 'services.serving.knative.dev'.
  • Registered a sidebar entry filter to hide the knative parent and all child entries if Knative is not installed on the active cluster.

Why this is needed

• UI Clutter: Statically registering Knative sidebar entries makes the menus permanently visible even on environments without Knative. Clicking them triggers dead views, leading to a cluttered and unpolished experience.
• Active Context Sync: It guarantees that the sidebar navigation dynamically adapts when switching between different Kubernetes clusters with different capabilities.

Steps to test

1. Switch to a Kubernetes cluster that does not have Knative installed.
2. Verify that the Knative sidebar menu entry is completely hidden.
3. Switch to a cluster with Knative installed.
4. Verify that the Knative sidebar menu entry (and all its sub-pages: KServices, Revisions, Domain Mapping, Cluster Domain Claims, Networking) displays correctly.

[Copilot]

Pull request overview

This PR adds cluster-aware sidebar filtering for the Knative plugin so the Knative menu can be hidden when the active cluster does not expose Knative Serving CRDs.

Changes:

• Adds CRD discovery state, TTL caching, and in-flight tracking for Knative installation checks.
• Registers a sidebar entry filter that hides the Knative parent and children when Knative is detected as absent.

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[Joshna907]

ptal now @illume