[FEATURE] Better integration of custom validators

[sypets]

goals:

• Do not show custom validators separately but show them all in one list and make it possible to customize order
  • example: have additional email validators, want to show them directly under the existing E-Mail validator
• Make it possible to override existing validators, e.g. email
• add interface for PHP validators

Currently, the existing validators are "hard-coded" , it is not easily possible to override them. The custom validators can be customized via TSconfig / TypoScript but not the existing validators:

If all validators were configured the same way (custom + native), I think this would automatically make this possible.

Main use case 1:

Add MX domain check to email validator (in PHP). It is possible to extend the JavaScript validator by just adding additional JavaScript, but I have not found a way to extend the PHP validation.

I have currently created a separate email validator for this, but it is confusing for the editors. I want them to just use the "email" validator.

Main use case 2:

Add additional email validator (for more restrictive fields which require only university emails). This should be displayed directly under the common email validator, not at the end of the list where it is easily overlooked.

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see Domain/Validator/InputValidator.php : isValidFieldInStringValidation