Accessing upstream using a http or https proxy

### Description

In many cases, where Apisix is running behind a corporate firewall, the only way to access some external or internal endpoints is through a http or https_proxy or sockes_proxy. Is it / how it is possible to define a proxy for an upsteam?

- `http_proxy`, `https_proxy`, `sockes_proxy`, `all_proxy` and `no_proxy` environment variables are ignored
- there's no proxy configuration option in `upstream`
- I couldn't found any documentation related to setting proxy, only in open_id_plugin, but looks like it's the only place

How are you dealing with proxies?

Could some documentation be added on the topic?

### Environment

- APISIX 3.5 Docker
- Operating system Windows and Linux

Start by reviewing APISIX upstream configuration and the existing open_id_plugin proxy documentation, then trace how the listed proxy environment variables are handled in the APISIX 3.5 Docker setup. Done means the supported proxy behavior and configuration path are verified and documented, or the missing support is clearly scoped with a reproducible test.

Written by the indexing model from the issue text.