feat: Add spec.trafficDistribution field for service

Add traffic distribution configuration to optimize Inter-AZ data
transfer costs by enabling zone-aware traffic routing. This change
introduces a new trafficDistribution field that allows configuring
traffic locality preferences, zone affinity rules, and cross-AZ
traffic minimization strategies. The implementation reduces cloud
egress charges by prioritizing same-zone service-to-service
communication while maintaining high availability through intelligent
failover to other availability zones when local resources are
unavailable.

Reference:
https://kubernetes.io/docs/concepts/services-networking/service/#traffic-distribution

Signed-off-by: younsl <cysl@kakao.com>

Author: younsl

charts/docker-mailserver/templates/service.yaml

@@ -145,3 +145,6 @@ spec:
   {{- if .Values.service.clusterIp }}
   clusterIP: {{ .Values.service.clusterIp }}
   {{- end }}
+  {{- if .Values.service.trafficDistribution }}
+  trafficDistribution: {{ .Values.service.trafficDistribution }}
+  {{- end }}

charts/docker-mailserver/values.yaml

@@ -300,6 +300,11 @@ service:
   ## Set it to "Local" when used with type "LoadBalancer" and set it to "Cluster" when used with "NodePort",
   ## unless you have a good reason not to.
   # externalTrafficPolicy: "Cluster"
+  ## Traffic distribution preference for Services. One of:
+  ## - PreferClose (routes traffic to endpoints that are topologically proximate)
+  ## This field requires Kubernetes 1.31+ and is only available when the feature gate is enabled.
+  ## See: https://kubernetes.io/docs/concepts/services-networking/service/#traffic-distribution
+  trafficDistribution: ""
   ## If there is a port associated with a given service, expose it here.
   # port:
   ## If there is a particular IP that should be used for the service, specify it here.