Add --cache-sync-timeout flag to make CacheSyncTimeout configurable

[itaibeny]

Issue

Fixes #4855

Description

On large clusters (~12,000 Services, ~2,000 HTTPRoutes) with ALBGatewayAPI=true, the Gateway API controller's Service informer cache cannot sync within the hardcoded 2-minute CacheSyncTimeout from controller-runtime. This causes a crash-loop on startup:

failed to wait for gateway.k8s.aws/alb caches to sync kind source: *v1.Service:
timed out waiting for cache to be synced for Kind *v1.Service

The timeout is already configurable in controller-runtime via ctrl.Options.Controller.CacheSyncTimeout (see pkg/config/controller.go), but the LBC never exposes it. This PR wires it through:

1. pkg/config/runtime_config.go — Add CacheSyncTimeout field to RuntimeConfig, register --cache-sync-timeout flag via BindFlags() (default: 2m, preserving current behavior), and pass it into opt.Controller.CacheSyncTimeout in BuildRuntimeOptions().
2. pkg/config/runtime_config_test.go — Unit tests for the flag default, custom values, and the wiring into ctrl.Options.
3. docs/deploy/configurations.md — Add cache-sync-timeout to the CLI flags table.
4. helm/aws-load-balancer-controller/values.yaml — Add cacheSyncTimeout value (empty = use controller default).
5. helm/aws-load-balancer-controller/templates/deployment.yaml — Conditional --cache-sync-timeout arg.

Why this is safe: default behavior is unchanged (2 minutes), no new dependencies, the field already exists in controller-runtime — we're just wiring it through.

Checklist

• Added tests that cover your change (if possible)
• Added/modified documentation as required (such as the README.md, or the docs directory)
• Manually tested
• Made sure the title of the PR is a good description that can go into the release notes

[kubernetes-prow]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: itaibeny
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[kubernetes-prow]

Hi @itaibeny. Thanks for your PR.

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