fix: preserve coredns config during cluster restart

[cjc7373]

What

fixes #1221

Why

Since coredns configmap will be reverted to original state during k3s startup process, this PR utilizes coredns-custom configmap (introduced in k3s-io/k3s#4397) to persistent custom coredns configs (e.g. the host.k3d.internal record).

Since coredns's host plugin can only be used once per server block, I have to use the file plugin.

A go template is used to render the configmap. I don't know if there's a better way.

Implications

Not I'm aware of.