[7.x] validate email with custom class

[Gummibeer]

The underlying egulias/email-validator package allows to add custom validators by implementing the Egulias\EmailValidator\Validation\EmailValidation interface.
But right now it's not possible to use the existing email rule to call these custom validators.

This PR allows to pass in a FQCN which is resolved by the container and returned.

use App\Validation\Email\MyCustomValidator;

Validator::make(
  ['email' => 'foo@bar.de'], 
  ['email' => 'email:rfc,dns,'.MyCustomValidator::class]
)->passes();

This example will call the rfc and dns validator like before but also call MyCustomValidator.
That's useful if you want to limit registration to a given email domain, filter trashmails or whatever you want to add on top.