Publicize or provide a function to get system proxy

[cnlancehu]

Currently, reqwest provides a good implementation for retrieving system proxies, but it is not exposed publicly. This limitation means developers can only set no_proxy but cannot easily check if a user has set a system proxy, which can be inconvenient.

It would be very helpful if reqwest could either:

1. Make the system proxy retrieval function public.
2. Provide a dedicated function that allows developers to easily check if a system proxy is set.

This feature would allow developers to determine whether a user is using a system proxy and make decisions accordingly, especially in sensitive scenarios where the use of a proxy might be relevant.

Thank you for considering this enhancement.