Add zonal affinity support

[08volt]

Overview

This PR implements zonal affinity capabilities for L4 Internal Load Balancers, allowing traffic to stay within zones when possible for improved performance and reduced cross-zone data transfer costs.

Changes

• Add zonal affinity flag for L4 ILB controllers
• Implement backendService zonal affinity control logic
• Add backendService minimum API version control for zonal features
• Update backendService tests to include zonal affinity cases with expected NetworkPassThroughLbTrafficPolicy
• Add handler for service spec Spec.TrafficDistribution change events
• Add L4 TestZonalAffinity integration tests

Usage

Users can enable zonal affinity by configuring their service with Spec.TrafficDistribution=PreferClose.

Testing

• Added unit tests for backendService with zonal affinity configurations
• Added L4 TestZonalAffinity integration tests to verify controller functionality

[k8s-ci-robot]

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This pull-request has been approved by: 08volt
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[k8s-ci-robot]

Hi @08volt. Thanks for your PR.

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