feat: add multi-container support to apexkube helm chart (#145)

[Luv7804]

Description:

Summary

Add additionalContainers support to the apexkube helm chart, enabling sidecar/multi-container pod deployments. This aligns with the existing implementation in the kube-oidc-proxy chart (see #140).

Changes

charts/apexkube/values.yaml

• Added serviceTemplate.additionalContainers: [] with commented usage example
• Supports both serviceTemplate.additionalContainers (applied to all services) and per-service additionalContainers overrides

charts/apexkube/templates/service.yaml

• Added additionalContainers rendering block after the healthcheck section and before dnsConfig
• When both serviceTemplate.additionalContainers and per-service additionalContainers are defined, they are stacked together
• Bug fix: Fixed nil pointer error in healthcheck condition, changed .healthcheck.enabled to and .healthcheck .healthcheck.enabled to prevent template failure when .healthcheck is nil