feat(ingress-controller): add imagePullSecrets support with global fallback

[ArnaudCHT]

What

Add deployment.imagePullSecrets support in ingress-controller with fallback to global.imagePullSecrets.

Why

The ingress-controller deployment had no way to configure imagePullSecrets, forcing users to manually patch deployments or fork the chart.

Changes

• Add deployment.imagePullSecrets configuration option
• Falls back to global.imagePullSecrets if not set locally
• Backward compatible and works standalone

https://github.com/apache/apisix-helm-chart/issues/883

Testing

To test this PR, you can use the following commands:

# Test with global.imagePullSecrets
helm template . -s templates/deployment.yaml \
  --set 'global.imagePullSecrets[0].name=global-registry-secret'

# Test with deployment.imagePullSecrets (local override)
helm template . -s templates/deployment.yaml \
  --set 'deployment.imagePullSecrets[0].name=my-registry-secret'

# Test priority: deployment takes precedence over global
helm template . -s templates/deployment.yaml \
  --set 'global.imagePullSecrets[0].name=global-registry-secret' \
  --set 'deployment.imagePullSecrets[0].name=my-registry-secret'

# Test standalone (no imagePullSecrets)
helm template . -s templates/deployment.yaml