Fetch: added forward proxy support with HTTPS tunneling.

[xeioex]

This patch adds forward proxy support to the ngx.fetch() method in njs, enabling HTTP and HTTPS requests through a proxy server with optional
authentication.

Key Features:

• HTTP proxying: Uses absolute-form URIs when sending requests through proxy
• HTTPS tunneling: Implements CONNECT method for HTTPS requests through proxy
• Proxy authentication: Supports Basic authentication via Proxy-Authorization header
• Keepalive support: Works with existing js_fetch_keepalive directive

New Directives:

• js_fetch_proxy - Configures forward proxy URL

Example nginx.conf:

...
  http {
      js_import script.js;

      server {
          listen 8080;
          
          resolver   127.0.0.1:5353;

          # Configure forward proxy
          location /api {
              js_fetch_proxy http://user:[email protected]:3128;
              js_content script.fetch_data;
          }
      }
  }

JavaScript usage:

  async function fetch_data(r) {
      // Will automatically use configured proxy
      let reply = await ngx.fetch('https://api.example.com/data');
      let body = await reply.text();
      r.return(200, body);
  }
  
 export default {fetch_data};
  

[VadimZhestikov]

Technically looks good, except for few comments.

May be it would be good discuss solution with Ivan, because it is bit different from #956