Allow to connect to DoH through proxy?

### 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

I live in China, but some stupid ISP blocked my server's connection to those doh servers that lie in overseas, so i always need to enable the TUN mode of V2RayN, but i dont think its a good solution.

btw i cant update the filters through your server (adguardteam.io) unless i get v2rayn's TUN mode open, otherwise it keeps connection failure!

### Proposed solution

When sending an HTTP request, can you allow adguardhome to connect to a proxy or just follow system proxy settings?

### Alternatives considered and additional information

_No response_