Fetch: added forward proxy support with HTTPS tunneling.

Fetch: added forward proxy support with HTTPS tunneling.

Supports Basic authentication via Proxy-Authorization header.

- js_fetch_proxy - configures forward proxy URL. It takes proxy URL
    as a parameter. The URL may optionally contain user and password.
     Parameter value can contain variables. If value is empty,
     forward proxy is disabled.

example.conf:
...
  http {
      js_import main.js;

      server {
          listen 8080;

          resolver   127.0.0.1:5353;

          location /api {
              js_fetch_proxy http://user:pass@proxy.example.com:3128;
              js_content main.fetch_handler;
          }
      }
  }

This implements #956 feature request on Github.