Add support for Secret kind in BackendTLSPolicy's spec.validation.caCertificateRefs[*].kind

[pmalek]

Problem statement

Support for BackendTLSPolicy in 3.4 introduced support for ConfigMap kind in spec.valiadation.caCertificateRefs.

This introduces a gap in implementation between using BackendTLSPolicy and using konghq.com/ca-certificates-secret or konghq.com/ca-certificates-configmap annotations on Service.

ℹ️ Only ConfigMap kind is considered Core as per the Gateway API spec: https://github.com/kubernetes-sigs/gateway-api/blob/26051f587ccaf54823ccfd7eca5cbf2013aa8177/apis/v1alpha3/backendtlspolicy_types.go#L115-L119

Proposed solution

Extend the code in

kubernetes-ingress-controller/internal/controllers/gateway/backendtlspolicy_controller.go

Lines 304 to 317 in 9f0ded8

	// Update references to ConfigMaps in the dataplane cache.
	referredConfigMapNames := listConfigMapNamesReferredByBackendTLSPolicy(backendTLSPolicy)
	if err := ctrlref.UpdateReferencesToSecretOrConfigMap(
	ctx,
	r.Client,
	r.ReferenceIndexers,
	r.DataplaneClient,
	backendTLSPolicy,
	referredConfigMapNames,
	&corev1.ConfigMap{}); err != nil {
	if apierrors.IsNotFound(err) {
	return ctrl.Result{Requeue: true}, nil
	}
	}

to also handle Secrets

Acceptance criteria

• As a user I can use Secret in BackendTLSPolicy's spec.validation.caCertificateRefs[*].kind with KIC.