DNS64: Only return synthetic AAAA results when querying a specific upstream DNS server

### 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

Hi, there.

I'm using AdguardHome v0.107.72 and it's been very helpful.

I'm using different upstream DNS servers for different network environments, and would like to enable DNS64 function on a specific (definitely NOT ALL) upstream DNS server.

However, AAAA results are synthesized regardless of what upstream DNS server is queried, and there is no settings or a way to control that behavior.

### Proposed solution

Add a mechanism to control the synthesis of AAAA results based on the upstream DNS server in empty AAAA results.

Add a flag in the AdguardHome configuration file for the control.

### Alternatives considered and additional information

_No response_