Add callable attributes support

[talelmishali]

This PR adds support for callable attributes.

Onboard::addStep('Payments')
    ->cta('Complete')
    ->attributes(function (User $model) {
            return [
                'link' => $model->stripeOnboarding(),
            ];
    });

My use case

I came across a need for it on a project I am currently working on; one of the attributes (link, in my case) is a Stripe onboarding link (returned from a model method). Therefore, I can not simply use the route() helper or static address.

[riasvdv]

Thanks!