Improve REST authentication method when registering fields

[JoryHogeveen]

Instead of checking if the user is logged in it also checks if the user a basic auth for applications is used.

Description

Fixes #7340

@sc0ttkclark I'm not 100% sure this will be the best method. You could also already set the current WP user so WP core doesn't revalidate using the same method. I'll leave this up to your judgement.

Testing instructions

See #7340

Try the same with this patch and see it now works!

PR checklist

• I have tested my own code to confirm it works as I intended.
• My code follows the WordPress Coding Standards.
• My code follows the WordPress Inline Documentation Standards.

[what-the-diff]

PR Summary

• Introduction of a New Validation Method
  A new method, is_rest_authenticated(), has been included in the PodsRESTFields class. This will help in verifying if a user or an application is properly logged in.

• Method Replacement for Enhanced Security
  The method field_allowed_to_extend in the PodsRESTFields class, previously using is_user_logged_in(), will now utilize is_rest_authenticated(). This change ensures a more secure and accurate validation process.

• Enhancement in Test Cleanup
  An update to the PodsRESTFieldsTest class has been made. After each test, the tearDown method will now invoke pods_static_cache_clear(). This helps in maintaining a clean slate for subsequent tests, improving the accuracy and reliability of the results.