Cloudflare Turnstile support

[amroland]

Description
Currently, there is no (native) support for integrating modern CAPTCHA solutions such as Cloudflare Turnstile into Sensei LMS frontend login (and related registration) forms.

Problem

• Google reCAPTCHA has become paid and raises privacy concerns.
• Cloudflare Turnstile is increasingly adopted as a privacy-friendly alternative.
• Existing Turnstile plugins typically hook only into the default WordPress and WooCommerce login and registration flows.
• Sensei LMS frontend login forms are custom and shortcode-generated, so they are not protected by these plugins.

As a result, Sensei LMS frontend login pages cannot easily be protected against bots and brute-force attempts using Turnstile.

Expected behavior
Sensei LMS should provide a way to protect its frontend login and registration forms with Cloudflare Turnstile.

Additional context
Other LMS platforms have already added Turnstile support. Providing first-class or extensible support in Sensei LMS would significantly improve security and developer experience.