User should be allowed to specify encrypted DNS servers

[cathyjf]

CoreDNS fully supports encrypted DNS remotes (https://coredns.io/plugins/tls/), but we don't expose this feature to the user (except that the fallback DNS configuration in Home Assistant OS relies on this functionality (namely, querying Cloudflare securely)). It would not be difficult to allow users to specify arbitrary encrypted DNS servers to query, but currently we allow users to specify only plain unencrypted DNS servers.

Note that users should be able to specify the hostname and IP of the secure remote, not just one or the other, so that a bootstrap server is not required to resolve the hostname. Again, CoreDNS already supports this feature, it's just not exposed in our user-facing config options.