Swift SDK Generator cannot download files via an HTTP proxy

[euanh]

Some environments, such as on CI servers, only allow HTTP requests to the Internet via an HTTP proxy. SDK Generator does not currently support using a proxy, so it cannot work in these environments.

Proxy addresses are often configured through several well-known environment variables such as http_proxy, HTTPS_PROXY and so on. SDK Generator should use these variables if present, and also offer command line arguments to configure or ignore proxies.