Use different upstream files for different clients

### Problem Description
I set up different upstreams for different clients in the client settings, but I can't use [upstream_dns_file](https://github.com/AdguardTeam/AdGuardHome/wiki/Configuration#loading-upstreams-from-file) here. Unfortunately, I didn't find the relevant instructions. If this feature already exists, please point it out for me.

### Proposed Solution
Hope to be able to use the the [upstream_dns_file](https://github.com/AdguardTeam/AdGuardHome/wiki/Configuration#loading-upstreams-from-file) in the client settings. So that we can use different upstream files for different clients