Add an option to auto-update the upstreams list

Apparently, this feature would be useful to the users from China since they use huge lists of upstream servers (`upstream_dns_file`) and would like to keep them up-to-date:
https://github.com/AdguardTeam/AdGuardHome/discussions/4305

The simple solution would be to allow specifying URLs in `upstream_dns_file`:
```
upstream_dns_file: "https://example.org/upstreams.txt"
```

We'll also need to somehow specify the refresh time. Could be even encoded into the URL itself (`refresh=5s; https://example.org/upstreams.txt`).

Other ideas are welcome.

I am a bit reluctant to implement this functionality right now, let's see if it gets upvoted.