support for http proxies in awc

In reqwest, when a system proxy is configured, it is used by default: https://docs.rs/reqwest/latest/reqwest/struct.ClientBuilder.html#method.proxy
In awc, it does not seem possible to configure an http proxy, and the environment variables HTTP_PROXY / HTTPS_PROXY are ignored by default: https://docs.rs/awc/latest/awc/index.html

It would be nice if awc supported proxies. Currently, any desktop software that depends on awc is hard to deploy in constrained/managed environments that require the usage of proxies to access the internet.

Initially requested in https://github.com/sqlpage/SQLPage/discussions/1269