Add a way to configure DNS stubDomains

[agodard]

Problem
I currently need to update my DNS configmap manually after installing my cluster. Could we implement a rke_cluster attribute to configure DNS stubDomains programmatically ?

See cluster DNS configuration : https://registry.terraform.io/providers/rancher/rke/latest/docs/resources/cluster#dns

Example

resource "rke_cluster" "my_cluster" {
  [...]
  dns {
    provider = "kube-dns"
    stubdomains = {
      "exemple.com": ["ns1.exemple.io", "ns2.exemple.org"]
    }
  }
}