[12.x] Allow mass assignment for value object casting.

[AbdelElrafa]

Hey Taylor,

I ran into an issue this morning when trying to mass assign an attribute that's a cast that wasn't being guarded.

Given a model that uses a value cast object, that's not explicitly guarded:

class Person extends Model
{
    protected $guarded = ['id'];
    public $timestamps = false;
    protected $casts = [
        'address' => AsAddress::class,
    ];
}

I would expect to be able to mass assign the attribute:

Person::create(['address' => new AddressDto('123 Main St.', 'Anytown, USA')]);

However, this was throwing: Illuminate\Database\Eloquent\MassAssignmentException: Add [address] to fillable property to allow mass assignment on [Illuminate\Tests\Database\Person].

I tried to use the Model::handleDiscardedAttributeViolationUsing(...) method added in #44664 to handle that within my app, but wasn't able to since the value that was trying to be set wasn't being passed into the closure. That's probably another update needed to make that hook more useful.

After digging into how we determine if an attribute is isFillable() or not, I see that we're already checking if the attribute hasSetMutator or hasAttributeSetMutator so it felt natural to also check if the attribute isClassCastable.

[ghaskell]

🥳