Add Horizontal Pod Autoscaler (HPA) Support for Airflow API Server

[HsiuChuanHsu]

Description:

This pull request introduces Horizontal Pod Autoscaler (HPA) support for the Airflow API server in the Helm chart, enabling automatic scaling of API server pods based on resource utilization or custom metrics. The changes enhance the chart's flexibility and scalability, allowing users to configure autoscaling for the API server to handle varying workloads efficiently.

Changes:

1. Added HPA Configuration Template
   Created chart/templates/api-server/api-server-hpa.yaml to define the HPA resource for the API server.
2. Updated Values Schema
   Modified chart/values.schema.json to include a new hpa section under apiServer.
3. Updated Default Values
   Added HPA configuration in chart/values.yaml under apiServer.hpa with sensible defaults
4. Added Unit Tests:
   Introduced helm-tests/tests/helm_tests/apiserver/test_hpa_apiserver.py to validate HPA behavior

How to Test:

1. Enable HPA with apiServer.hpa.enabled=true and verify the HPA resource is created with default settings.
2. Override minReplicaCount, maxReplicaCount, metrics, or behavior in values.yaml and confirm the HPA resource reflects the changes.
3. Run unit tests with pytest helm-tests/tests/helm_tests/apiserver/test_hpa_apiserver.py to validate logic.

Additional Notes:

Included deployment and HPA status screenshot from kubectl displaying an Airflow API server deployment with 4 pods running.

Related Issues:

#51935

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[boring-cyborg]

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[HsiuChuanHsu]

Hi @romsharon98, @eladkal could you help review this PR? Thanks!

[github-actions]

This pull request has been automatically marked as stale because it has not had recent activity. It will be closed in 5 days if no further activity occurs. Thank you for your contributions.

[HsiuChuanHsu]

No!! Pls don't!!

[jscheffl]

Tanks for being patient with the PR. From review this looks good to me. Some nit in regards of docs as what the users will see - mainly to be clear.

Then if adjusted I assume good to merge, if not somebody objects.

[HsiuChuanHsu]

Happy to see my first PR got reviewed from you. Thanks.