api: Add AvailabilityZones field to LoadBalancerSpec

Add support for configuring availability zones on load balancers to enable
zone-redundant configurations for high availability.

For internal load balancers, zones are set directly on the frontend IP
configuration. For public load balancers, zones should be set on the
associated public IP addresses.

The field supports up to 3 zones and uses a set list type to prevent
duplicates. Zones are immutable after creation per Azure platform
requirements.

Fixes #5709

Author: bryan-cox

api/v1beta1/types.go

@@ -364,6 +364,14 @@ type LoadBalancerSpec struct {
 	// BackendPool describes the backend pool of the load balancer.
 	// +optional
 	BackendPool BackendPool `json:"backendPool,omitempty"`
+	// AvailabilityZones is a list of availability zones for the load balancer.
+	// When specified for an internal load balancer, the frontend IP configuration
+	// will be zone-redundant across the specified zones.
+	// For public load balancers, this should be set on the associated public IP addresses instead.
+	// +optional
+	// +listType=set
+	// +kubebuilder:validation:MaxItems=3
+	AvailabilityZones []string `json:"availabilityZones,omitempty"`
 
 	LoadBalancerClassSpec `json:",inline"`
 }