Configure upstream servers via Discovery of Designated Resolvers

The [Discovery of Designated Resolvers RFC draft](https://datatracker.ietf.org/doc/draft-ietf-add-ddr) describes the mechanism which allows automatically configure encrypted DNS to it's clients. Thus, it may also be used to automatically configure a set of upstream servers for AdGuard Home. Perhaps, we may use some custom URL scheme for this, for example `ddr://`:

```none
ddr://8.8.8.8
```
or
```none
ddr://dns.google
```

The above should make AdGuard Home to use all the encrypted resolvers `8.8.8.8` (`dns.google`) offers.