Add support polymorphic relations

In 2.0, it is very upsetting that there are no polymorphic relations out of the box. 
It would be cool to be able to use such relations.
It would be very nice to use something like this:

```php
return $this->hasMany(Comment::class, ['entity_id' => 'id'])
    ->polyTo(['entity_type'], static::class);
```
