Custom upstream file paths for persistent 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

Can a feature be added to AdGuard Home's client settings to allow custom DNS for a specific client, similar to the main DNS settings, it can support file paths?

### Proposed solution

Just as mentioned above，add dns file paths for client settings

### Alternatives considered and additional information

_No response_