Add support for DoH (DNS-over-HTTPS)

[sagebind]

DNS-over-HTTPS is useful in strong privacy scenarios to make encrypted DNS name lookups.

DoH doesn't replace the classic DNS protocol but rather adds a new layer on top of DNS resolution, so it should be a separate option that does not affect other DNS resolution configuration. Since DoH is not auto-configured (à la DHCP) in most networks, it will be strictly opt-in and we will require users to specify their own server adresses.