PHPORM-243 Alias _id to id in Schema::getColumns()

[GromNaN]

Fix PHPORM-243

❯ php artisan model:show book

  App\Models\Book .............................................................  
  Database ............................................................ mongodb  
  Table ................................................................. books  

  Attributes ...................................................... type / cast  
- _id ..................................................... objectId / accessor  
+ id ...................................................... objectId / accessor   
  author nullable ...................................................... string  
  created_at nullable ......................................... date / datetime  
  description nullable ................................................. string  
  isbn nullable ........................................................ string  
  publication_date nullable ............................................ string  
  title nullable ....................................................... string  
  updated_at nullable ......................................... date / datetime  
-  id ................................................................. accessor  

Required for API Platform integration with Laravel: https://github.com/api-platform/laravel/blob/e821ad27072228e4638f3de92d51fc3d15e02c7b/Eloquent/Metadata/ModelMetadata.php#L68

Checklist

• Add tests and ensure they pass
• Add an entry to the CHANGELOG.md file

[alcaeus]

One nit regarding a comment, but LGTM other than that