Add support for exposing control plane health endpoint

[yugannkt]

Description

Adds support for exposing the control plane readiness probe endpoint (/readyz) to enable external load balancers like Azure Application Gateway to perform health checks.

Fixes #4400

Changes

• Added nginxGateway.service.exposeHealthPort Helm value (default: false)
• Updated control plane Service template to conditionally expose health port
• Generated Helm schema and documentation
• Added CHANGELOG entry

Usage

nginxGateway:
  service:
    exposeHealthPort: true

This exposes /readyz on port 8081 (or configured readinessProbe.port) for external load balancer health checks.

Checklist

Before creating a PR, run through this checklist and mark each as complete.

• I have read the CONTRIBUTING doc
• I have added tests that prove my fix is effective or that my feature works
• I have checked that all unit tests pass after adding my changes
• I have updated necessary documentation
• I have rebased my branch onto main
• I will ensure my PR is targeting the main branch and pulling from my branch from my own fork

Release notes

Add support for exposing control plane readiness probe endpoint for external load balancer health checks.

[nginx-bot]

Hi @yugannkt! Welcome to the project! 🎉

Thanks for opening this pull request!
Be sure to check out our Contributing Guidelines while you wait for someone on the team to review this.

[github-actions]

✅ All required contributors have signed the F5 CLA for this PR. Thank you!
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[yugannkt]

I have hereby read the F5 CLA and agree to its terms