Merging the coredns/multicluster plugin in tree

What would you like to be added: Merging the coredns/multicluster plugin (https://github.com/coredns/multicluster) in-tree/in the coredns main branch.

Why is this needed: The CoreDNS multicluster plugin is the implementation of KEP-1645 (https://github.com/kubernetes/enhancements/blob/master/keps/sig-multicluster/1645-multi-cluster-services-api/README.md#dns) and from what is publicly available is used by

AWS (https://github.com/aws/aws-cloud-map-mcs-controller-for-k8s but alpha though)
Alibaba (https://www.alibabacloud.com/help/en/ack/manage-multi-cluster-services-by-using-the-cli?spm=a2c63.p38356.0.0.445d6a42eAVFn4#uicontrol-8rm-imv-mer)
Cilium (https://docs.cilium.io/en/latest/network/clustermesh/mcsapi/#installing-coredns-multicluster which will be released in Cilium 1.17 in ~Jan/Feb of next year)
In e2e tests of the mcs-api repo (https://github.com/kubernetes-sigs/mcs-api/blob/eff1ba8c3ab2d50a7faa0a02d18dc6ef9f58a21d/scripts/up.sh#L52)

Outside of that the Kubernetes SIG handling this KEP (SIG Multicluster) is looking at graduating to beta in the coming months: https://github.com/kubernetes-sigs/mcs-api/issues/48.

On my side I represent mostly the Cilium side of things and we would aim to somehow simplify the installation process of the coredns multicluster plugin which triggered this proposal/question. We could start by providing a docker image with the multicluster plugin compiled in/included but unlike AWS and Alibaba we run on many different types of Kubernetes installation and we don't control tightly everyone's setup so this comes with the risk that the Kubernetes cluster has installed a specific version of CoreDNS with some additional plugins. Including this in-tree would increase significantly the chance that the multicluster plugin would be compiled-in in the long term in the coredns image already installed in the cluster.

I would be happy to do all the work of getting the plugin in-tree and any potential improvements in order to do that. And as Cilium is aiming to be conformant and follow closely the KEP I would be happy to work on the maintenance of the plugin to keep it up to date with the KEP.

Let me know if getting the plugin in-tree looks reasonable to you, if yes I would start drafting some PR/checking more in-depth how to do this. Thanks!

coredns / coredns

Merging the coredns/multicluster plugin in tree #6921