feat: make leader election lease duration configurable

[Dasmat13]

Closes #3823.

This PR implements a configurable leaseDurationSeconds for the AWS Load Balancer Controller by introducing an opt-in command-line flag (--leader-election-lease-duration) and updating the Helm chart values/templates/README.

Changes:

• Added --leader-election-lease-duration flag to the controller manager.
• Designed the flag to be strictly opt-in (omitting it retains controller-runtime's built-in defaults, as requested by maintainers).
• Added leaderElectionLeaseDuration to the Helm values and templates, and documented it in the README.
• Added unit tests in pkg/config/runtime_config_test.go to test the options building logic.

[kubernetes-prow]

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[Dasmat13]

Hi @oliviassss and @wweiwei-li 👋

When you have a chance, could you please take a look at this PR? It also needs /ok-to-test before the required CI jobs can run.

The implementation keeps the feature opt-in, so existing behavior remains unchanged unless --leader-election-lease-duration is explicitly configured. Happy to make any changes based on your feedback. Thanks!