Add Gateway API ingress support to kn operator plugin

[kahirokunn]

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Add Gateway API as a new ingress option for Knative Serving

This PR adds support for Gateway API as a fourth ingress option in the kn-operator plugin, alongside the existing Istio, Kourier, and Contour options. Gateway API is the successor to the Ingress API and is becoming the standard for traffic management in Kubernetes, making it an important networking option for Knative users.

The implementation follows the same pattern established by the existing ingress options:

• Adds a --gateway-api flag to both the enable ingress and install commands
• Sets the ingress class to gateway-api.ingress.networking.knative.dev when Gateway API is selected
• Configures the spec.ingress.gateway-api.enabled field in the KnativeServing CR via the YTT overlay templates
• Validates that only one ingress type is selected at a time, consistent with existing behavior
• Includes comprehensive test coverage for all new functionality, including conflict validation with other ingress types

When to use Gateway API

Consider using Gateway API if your cluster already has a Gateway API-compatible controller installed and you prefer the modern, standards-based approach to Kubernetes ingress management. Gateway API is designed to be the successor to the Ingress API and provides richer routing capabilities.

Prerequisites

• A Gateway API-compatible controller (e.g., Envoy Gateway, Istio, Contour) must be installed in your cluster
• Gateway API CRDs must be available in your cluster

Usage

# Enable Gateway API ingress for Knative Serving
kn-operator enable ingress --gateway-api --namespace knative-serving

# Install Knative Serving with Gateway API ingress
kn-operator install -c serving --gateway-api --namespace knative-serving

/kind enhancement

Gateway API is becoming the standard for traffic management in Kubernetes (GA since v1.0.0). Adding Gateway API support to the kn-operator plugin allows users to adopt this modern networking option when deploying Knative Serving.

Release Note

Users can now configure Gateway API as the networking layer for Knative Serving using the `--gateway-api` flag with the `enable ingress` and `install` commands, in addition to the existing Istio, Kourier, and Contour options.

Docs

The `enable ingress` and `install` commands now support a `--gateway-api` flag to configure Gateway API as the ingress for Knative Serving. This works the same way as the existing `--istio`, `--kourier`, and `--contour` flags. Only one ingress type can be selected at a time. A Gateway API-compatible controller must be pre-installed in the cluster.

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