Support to select DNS upstreams randomly or by round-robin

### Prerequisites

- [x] I am running the latest version
- [x] I checked the documentation and found no answer
- [x] I checked to make sure that this issue has not already been filed

### Feature Description

Allow AdGuard to select upstream DNS servers randomly(not weighted)

### Proposed Solution

Add another strategy option besides current load-balancing, parallel requests and fastest IP address.

### Additional Information

That's something supported in [dnscrypt-proxy](https://github.com/DNSCrypt/dnscrypt-proxy) and [blocky](https://0xerr0r.github.io/blocky), not sure if you'd also consider to support it in AdGuard Home? Thanks ;)