[8.x] Add the ability to transform paginator items

[reinink]

This PR adds the ability to transform paginator items via a new through() method, which returns an instance of the Paginator, and not the underlying Collection.

Here's an example use case:

return Inertia::render('Contacts/Index', [
    'contacts' => Contact::paginate()->through(function ($contact) {
        return [
            'id' => $contact->id,
            'name' => $contact->name,
            'phone' => $contact->phone,
            'city' => $contact->city,
            'organization' => optional($contact->organization)->only('name')
        ];
    }),
]);

This is needed since it's very common to want to map the data prior to the paginator being JSON encoded. However, right now there is no way to do that, since calling map() or transform() on the paginator object forwards calls to the underlying collection, which returns an instance of the Collection, not the Paginator.

Meaning, if you call $paginator->transform(), you'll only get the data back, and not the paginator toArray() structure (data, links, current_page, total, etc).

We could also call this transformItems(), but I think through() is more expressive.

[deleugpn]

I just built a component in front of JsonResource to do something pretty similar to this, but much more complex than this. I think I'll be able to simplify some of my APIs with this as well. Seems low-cost, high-benefit.