Allow DoH to use the Windows system proxy

### Issue Details

Please consider adding an optional setting that allows DoH upstream connections to use the Windows system proxy.

I understand that AdGuard currently bypasses the system proxy for DoH to avoid possible DNS/proxy dependency loops. This behavior can remain the default.

### Proposed solution

Add an option in **Advanced Settings**, disabled by default, with a warning about possible DNS loops.

Ideally:

- the main DoH connection can use the Windows system proxy;
- Bootstrap DNS remains direct;
- Fallback DNS remains direct.

This would allow advanced users to use proxied DoH without changing AdGuard's current safe default behavior.

### Alternative solution

_No response_