[7.x] Support returning an instance of a caster

[brendt]

With the addition of the Castable interface (laravel/docs#5934), it makes sense to allow direct instantiation from within a Castable::castUsing implementation. A common usecase could be something like this:

class EloquentDataTransferObject extends DataTransferObject implements Castable
{
    public static function castUsing()
    {
        return new DataTransferObjectCaster(static::class);
    }
}

Which allows a kind of "generic" cast. All classes extending EloquentDataTransferObject can now directly be used as a cast type, and will be casted to the correct implementation of EloquentDataTransferObject.

Eg.

class MyData extends EloquentDataTransferObject {
    public int $property = 1;
}

class ModelX extends Model
{
    protected $casts = [
        'data' => MyData::class,
    ];
}

[brendt]

If this gets merged, I'll also submit a PR targeting Laravel 8, which allows passing the arguments directly to public static function castUsing(array $arguments).

[taylorotwell]

Send this to master please. I already made changes to this on master.