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

[brendt]

See #32213 (comment)

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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]

@taylorotwell I suppose this means it'll be added in 8.x and not 7.x?

[mfn]

Would be great to have this in 7.x … it's backwards compatible, no?

[driesvints]

@brendt correct.

@mfn Taylor asked to send this to master instead.

[taylorotwell]

OK then send back to 7.x ... haha... I'll just merge it forward. Sorry for the inconvenience of making you send again. 😬

[brendt]

No problem, here it is: #32225