Option to disable DNS64 for specific clients

### Prerequisites

- [x] I have checked the [Wiki](https://github.com/AdguardTeam/AdGuardHome/wiki) and [Discussions](https://github.com/AdguardTeam/AdGuardHome/discussions) and found no answer

- [x] I have searched other issues and found no duplicates

- [x] I want to request a feature or enhancement and not ask a question

### The problem

VPN clients connecting to a home subnet cannot use an Adguard Home DNS server unless they want to route DNS64/NAT64 traffic through their VPN (breaking split tunnels essentially).

### Proposed solution

Implement a way to turn off DNS64 for specific subnets.

### Alternatives considered and additional information

The only way around this is to run 2 separate instances of AdGuard Home, one with DNS64 turned on, and the other turned off.