[FactoryCollection] add a way to index by a specific field

something like this could be useful:
```php
$users = UserFactory::new()
	 ->sequence([['username' => 'foo', 'username' => 'bar']])
	 ->indexBy('username')
	 ->create();
```

and then, we can access users by their username: `$users['foo`]`, `$users['bar']`