User registration: Enable extra field validation for user registration in Chamilo

[juancp-contidosdixitais]

This pull request introduces a new functionality to Chamilo that allows the use of a user extra field to validate user registrations. For instance, if we have an extra field to store passport numbers, we can ensure that there are no duplicate values.

This functionality also works on batch user registrations.

To utilize this functionality, you need to specify in "extra_fields_to_validate_on_user_registration" within configuration.php the extra fields you want to validate. For example:

$_configuration['extra_fields_to_validate_on_user_registration'] = [
'extra_fields' => [
'passport', 'employee_id'
]
];

[juancp-contidosdixitais]

Updated to add the same logic to the user add/edit actions from administration

[ywarnier]

I see the point of this PR, but the code comparison is a complete mess (probably not your fault - some mix of bad detections by Github and of adding changes to many small places, and maybe some parallel history of the same files in the last 5 months after your PR), so in the current state I cannot safely approve it (it would have to be rewritten with less intrusive changes).

[ywarnier]

This has been integrated (slightly differently) in 75ef745 in mid-2025