EKS Add-On CoreDNS] [request]: Support customising Corefile and using import plugin

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**Tell us about your request**
Support customising `Corefile` and using [`import`](https://coredns.io/plugins/import/) plugin.

**Which service(s) is this request for?**
EKS addon(CoreDNS)

**Tell us about the problem you're trying to solve. What are you trying to do, and why is it hard?**

I want to be able to split a large `Corefile` into a few smaller pieces and `import` them into the main `Corefile`.
Ideally, support whatever is there in the CoreDNS helm chart.

- [extraVolumes & extraVolumeMounts](https://github.com/coredns/helm/blob/3adf07751ed51ff3581bd4a776d950e0fde92364/charts/coredns/values.yaml#L249-L255)
- [servers & extraConfig](https://github.com/coredns/helm/blob/3adf07751ed51ff3581bd4a776d950e0fde92364/charts/coredns/values.yaml#L105-L164)
- [zoneFiles](https://github.com/coredns/helm/blob/3adf07751ed51ff3581bd4a776d950e0fde92364/charts/coredns/values.yaml#L232-L241)

**Are you currently working around this issue?**
Blocked my automation work.

**Additional context**

https://github.com/aws/containers-roadmap/issues/1275

**Attachments**

Start by reviewing the CoreDNS Helm chart values.yaml sections linked in the issue: extraVolumes and extraVolumeMounts, servers and extraConfig, and zoneFiles. Compare those capabilities with the EKS CoreDNS add-on, then verify that the requested Corefile customization and import-plugin support are available.

Written by the indexing model from the issue text.