feat: add ambient mode support to kubeflow controllers

[madmecodes]

Motivation

This PR makes the Kubeflow controllers (notebook-controller, tensorboard-controller, and pvcviewer-controller)
compatible with Kubernetes Gateway API, which is required for Istio ambient mode support.

Why this change is needed

Istio ambient mode is the service mesh architecture that provides simplified operations and
improved performance. However, it requires different routing resources compared to traditional Istio sidecar
mode:

• Sidecar mode (current): Uses Istio-specific VirtualService resources for L7 routing
• Ambient mode (new): Uses Kubernetes-native HTTPRoute resources from the Gateway API for L7 routing

What this PR accomplishes

1. Adds Gateway API support: Controllers can now create and manage HTTPRoute resources alongside existing
   VirtualService support
2. Maintains backward compatibility: Default behavior remains unchanged (sidecar mode with VirtualService)
3. Proper separation of concerns: USE_ISTIO controls VirtualService creation, USE_GATEWAY_API controls HTTPRoute
   creation
4. Configurable deployment: Environment variables allow switching between modes without code changes

Configuration

• Sidecar mode (default): USE_ISTIO=true, USE_GATEWAY_API=false
• Ambient mode: USE_ISTIO=false, USE_GATEWAY_API=true
• Gateway configuration: K8S_GATEWAY_NAME and K8S_GATEWAY_NAMESPACE specify the target Gateway resource

This change enables Kubeflow to work seamlessly in both traditional sidecar environments and modern ambient mesh
deployments.

Reference: kubeflow/kubeflow#7736

[google-oss-prow]

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[juliusvonkohout]

@kimwnasptd @orfeas-k for review.