[12.x] Add a new model cast named asFluent

[azim-kordpour]

Hi,
Since the Fluent class was added to Laravel, I have been using the Fluent cast on some of my applications where I need to deal with JSON values in different ways. I think it would be great to have it in the framework.

Example:

namespace App\Models;

use Illuminate\Database\Eloquent\Casts\AsFluent;
use Illuminate\Database\Eloquent\Model;

class Passenger extends Model
{
    protected function casts(): array
    {
        return [
            'address' => AsFluent::class,
        ];
    }
}

It can be used:

 $passanger = Passenger::first();
 
 $passenger->address->get('office.street');
 $passenger->address->get('home.street');

It can be updated by passing the fluent object or array

use Illuminate\Support\Fluent;

 $passanger = Passenger::first();
 
 $passenger->address = new Fluent([
           'office' => ['street' => 'new street'],
           'office' => ['home' => 'new street']
])
 
 // by array
 $passenger->address = [
           'office' => ['street' => 'new street'],
           'office' => ['home' => 'new street']
]

I tried to follow the naming convention. Please let me know if there's anything wrong or if I can improve it.