SSRF and local file disclosure via unvalidated URLs

Summary

Applications that pass attacker-controlled URLs to `webpage` versions before 2.1.0 may be vulnerable to server-side request forgery (SSRF) or local file disclosure.

`HTTP::fetch` and `Webpage::from_url` passed URLs to libcurl without restricting the URL scheme or validating the resolved destination IP address. Redirects were followed without checking whether the new destination was internal. Depending on how the application uses the response, an attacker could:

- Read local files accessible to the application process using `file://` URLs. - Cause requests to internal or link-local services (e.g. cloud metadata endpoints at `http://169.254.169.254`) via direct private-IP URLs or by chaining HTTP redirects through an attacker-controlled server.

Starting in 2.1.0, `HTTP::fetch` and `Webpage::from_url` reject non-HTTP(S) URL schemes and destinations that resolve to non-public IP addresses, including after redirects. Applications that intentionally fetch trusted internal resources can opt in to the previous behavior with `HTTP::fetch_unrestricted` or `Webpage::from_url_unrestricted`.

Reported by [Aiden Mohan](https://github.com/AidenMohan).