Support custom validation rules via classes

[simonrjones]

Custom packages in Laravel can have validation rules, which are called via a class. This does not currently work in the Form builder.

E.g. in app/Forms/BookRequestForm.php

        ->add('isbn', TextType::class, [
            'label' => 'ISBN',
            'rules' => ['required', new Isbn([13])]
        ])

This does work, which is only OK since a shortcode has been setup for this rule. This may not always exist in custom validation rules.

        ->add('isbn', TextType::class, [
            'label' => 'ISBN',
            'rules' => ['required', 'isbn:13']
        ])

See https://laravel.com/docs/13.x/validation#custom-validation-rules

[gtrinkwon]

We can use a service provider to register any custom rules of our own with string names, so they can be invoked as strings rather than new Rule()

However for any custom rules that do not do this, this might prove to be a problem.