Add DDR support to plain DNS upstream

Currently, it only supports it on the server-side, not on the client-side.

Basically, it means that we should provide an option that will automatically upgrade plain DNS to one of the encrypted protocols provided in DDR response.

Note, that this is a low-priority feature request. The way dnsproxy is currently used assumes that the user knows what exact protocol they want to use and auto-upgrade might be unexpected.