Security: Prevent SSRF attacks

SSRF (Server Side Request Forgery) vulnerability allows an attacker to change a parameter used on the Node.js application to create or control requests from the vulnerable server.

This introduces attack vectors such as
- scanning the internal network
- timeout the thread
- bypass host based authentication
- sending requests impersonating the server

The example could show the use of a whitelist of allowed domains and protocols from where the Node.js can fetch remote resources (and mention to avoid the use of user provided url's unless really required)