What type of PR is this?

feat(api): add Kubernetes client-go style typed clients for Envoy Gateway resources

What this PR does / why we need it:

This PR adds Kubernetes-style typed clientsets, informers, and listers for Envoy Gateway's custom resources, enabling external users to programmatically interact with Envoy Gateway resources using familiar patterns from kubernetes/client-go.

Key Changes:

1. Client Generation Infrastructure

• Added client-gen, lister-gen, and informer-gen from k8s.io/code-generator to tools
• Created tools/src/update-codegen.sh script to generate typed clients
• Created tools/src/verify-codegen.sh script to verify generated code is up-to-date
• Integrated generation into Makefile with make kube-generate-clients and make kube-verify-clients

2. API Annotations

Added code-generator annotations to 8 CRD types in api/v1alpha1/:

• Backend and BackendList
• BackendTrafficPolicy and BackendTrafficPolicyList
• ClientTrafficPolicy and ClientTrafficPolicyList
• EnvoyExtensionPolicy and EnvoyExtensionPolicyList
• EnvoyPatchPolicy and EnvoyPatchPolicyList
• EnvoyProxy and EnvoyProxyList
• HTTPRouteFilter and HTTPRouteFilterList
• SecurityPolicy and SecurityPolicyList

Annotations added:

• +genclient - Marks types for client generation
• +k8s:deepcopy-gen:interfaces=k8s.io/apimachinery/pkg/runtime.Object - Ensures runtime.Object interface compliance
• +k8s:deepcopy-gen=package - Enables package-level deepcopy generation in doc.go

3. Scheme Registration Updates

Updated api/v1alpha1/groupversion_info.go:

• Modified SchemeBuilder to use runtime.NewSchemeBuilder for code-generator compatibility
• Introduced localSchemeBuilder for controller-runtime compatibility
• Added SchemeGroupVersion alias for generated client compatibility
• Updated addKnownTypes to register all 8 CRD types and their list types

4. Generated Client Code (49 files)

Clientsets (pkg/client/clientset/versioned/):

• Versioned clientset with discovery support
• Typed clients for all 8 resources
• Fake clientset for testing

Informers (pkg/client/informers/externalversions/):

• Shared informer factory
• Per-resource informers with watch/cache capabilities
• Efficient event handling for controllers

Listers (pkg/client/listers/api/v1alpha1/):

• Cache-based listers for all resources
• Namespace-scoped and cluster-scoped list operations

5. Testing

Added pkg/client/clientset_test.go:

• Tests clientset instantiation
• Tests fake clientset for user testing scenarios
• Verifies all resource clients are accessible

6. CI/CD Integration

• Added .github/workflows/verify-codegen.yaml workflow to enforce generated code is up-to-date
• Updated .github/codecov.yml to exclude generated client code from coverage reports
• Integrated verification into existing gen-check workflow

Benefits:

1. Familiar API: Users can interact with Envoy Gateway resources using the same patterns as core Kubernetes resources
2. Type Safety: Compile-time type checking for all operations
3. Efficient Watching: Built-in informers provide efficient caching and event handling
4. Controller Support: Enables building external controllers and operators
5. Testing Support: Fake clients for unit testing
6. Production Ready: Follows kubernetes/client-go conventions

Example Usage:

import (
    "context"
    metav1 "k8s.io/apimachinery/pkg/apis/meta/v1"
    "github.com/envoyproxy/gateway/pkg/client/clientset/versioned"
)

// Create a clientset
clientset, err := versioned.NewForConfig(config)
if err != nil {
    panic(err)
}

// List SecurityPolicies
policies, err := clientset.GatewayV1alpha1().
    SecurityPolicies("default").
    List(context.TODO(), metav1.ListOptions{})

// Watch for changes using informers
informerFactory := informers.NewSharedInformerFactory(clientset, time.Minute)
policyInformer := informerFactory.Gateway().V1alpha1().SecurityPolicies()

policyInformer.Informer().AddEventHandler(cache.ResourceEventHandlerFuncs{
    AddFunc: func(obj interface{}) {
        policy := obj.(*v1alpha1.SecurityPolicy)
        fmt.Printf("SecurityPolicy added: %s\n", policy.Name)
    },
})

Design Decisions:

1. No applyconfiguration-gen: Not needed yet; can be added when users request Server-Side Apply support
2. No openapi-gen: controller-gen already generates OpenAPI schemas in CRDs (modern approach)
3. No conversion-gen: Only have v1alpha1; will add when promoting APIs to v1beta1/v1
4. Manual scheme registration: More explicit than register-gen; suitable for <10 API groups

This approach follows kubernetes/sample-controller and matches what other projects like cert-manager and external-dns use.

Which issue(s) this PR fixes:

Fixes #

Release Notes: Yes

# release-notes/current.yaml
feature:
  - area: api
    change: |
      Added Kubernetes-style typed clientsets, informers, and listers for programmatic access to Envoy Gateway resources.
      External users can now interact with Envoy Gateway CRDs using familiar client-go patterns.
    prNumber: <7356>

Checklist:

• Code generated using k8s.io/code-generator tools (client-gen, informer-gen, lister-gen)
• All 8 CRD types annotated with +genclient tags
• Scheme registration updated for both controller-runtime and code-generator compatibility
• Generation scripts created (update-codegen.sh, verify-codegen.sh)
• Makefile targets added (kube-generate-clients, kube-verify-clients)
• CI workflow added to enforce generated code is up-to-date
• Basic smoke tests added (pkg/client/clientset_test.go)
• Generated code excluded from code coverage
• Documentation inline with generated code
• Follows kubernetes/client-go conventions