Add support for HTTP_PROXY/HTTPS_PROXY/NO_PROXY environment variables
to the awc HTTP client, matching the behavior of reqwest and other
HTTP clients. Proxied requests use absolute-form URIs per RFC 7230
section 5.3.2.
Changes:
- actix-http: Add PROXY_REQUEST flag and absolute-form URI encoding
- awc: Add ProxyConfig, ClientBuilder::proxy()/no_proxy() methods
- awc: DefaultConnector routes connections through proxy when configured
- Proxy env vars are read by default; use no_proxy() to disable