Feature request: Add Cloudflare Turnstile support as an alternative to reCAPTCHA

### Is your feature request related to a problem? Please describe
Yes. WP Job Manager currently supports reCAPTCHA, but there does not seem to be a built-in option to use Cloudflare Turnstile instead.

We are using WP Job Manager for job applications and have recently received an unusual number of spam/fake applications. Many of them appear to be automated, generic, or AI-generated.

reCAPTCHA is currently enabled, but on other forms we are moving away from Google reCAPTCHA and using Cloudflare Turnstile instead. It would be helpful if WP Job Manager could support Turnstile as an alternative CAPTCHA provider.

### Describe the solution you'd like
Add Cloudflare Turnstile support as an alternative anti-spam option in WP Job Manager.

### Describe alternatives you've considered
We have also considered adding Turnstile manually with custom code or using a separate form plugin for applications, but native support in WP Job Manager would be cleaner and easier to maintain.

### Additional context
Google is currently moving reCAPTCHA Classic towards the Google Cloud / reCAPTCHA Enterprise setup. Because of these changes, some WordPress site owners may look for alternatives to Google reCAPTCHA.

Cloudflare Turnstile is already a common alternative and would be useful for WP Job Manager users who want spam protection for job/application forms without relying exclusively on reCAPTCHA.

Start by tracing the existing reCAPTCHA integration in the repository, including how it is configured and applied to job application forms. Compare those requirements with Cloudflare Turnstile's integration needs; done means Turnstile is available as an alternative anti-spam provider without removing reCAPTCHA support.

Written by the indexing model from the issue text.