Add support for DNS64/NAT64 networks

The problem is that using a custom DNS server in the IPv6-only T-Mobile network is impossible as it relies on their own default DNS in the following way: when a domain is hosted on an IPv4 address, their DNS returns a private (?) IPv6-mapped address instead.

1. For instance, `example.org` address is `93.184.216.34`.
2. T-Mobile's DNS server will resolve it to something like this instead: `::FFFF:93.184.216.34` (and this address is reachable in their IPv6-only network).

If we figure what exact prefix they use for IPv6 mapping (most likely it is `::FFFF:` but I need a confirmation), we will be able to convert IPv4 addresses returned by a custom DNS server to their IPv6-mapped counterpart.

Please note, that this is just an assumption.

**We need help from users who are on T-Mobile to figure if that is so.** If you are a T-Mobile user, please collect AdGuard logs and send them to `devteam at adguard.com`.