Add proxy support for DNS upstreams (DOH/DOT/(plain DNS?/DoQ?))

### Prerequisites

Please answer the following questions for yourself before submitting an issue. **YOU MAY DELETE THE PREREQUISITES SECTION.**

- [y] I am running the latest version
- [y] I checked the documentation and found no answer
- [y] I checked to make sure that this issue has not already been filed

### Problem Description

As I use DoH to avoid the dns pollution, but from mainland china, it's very slow to use https://1.1.1.1/dns-query or https://8.8.4.4/dns-query, or it's just blocked sometimes. So I'd like the doh requests can use the proxy, just like dnsmasq:
```
environment:
- http_proxy=socks5://192.168.8.188:3388
```

I tested the docker-compose environment variable, it doesn't work, either for the github update or doh requests. Then I put the proxy to guardhome's yaml config file, seems the github update uses it, but doh still doesn't use it.
It'll much faster and stable if doh can use a given proxy. Thank you very much.

### Proposed Solution

Let DoT and DoH use the given proxy, from the yaml config file or from the environment variable which is very flexible.