Fix return types

[madrussa]

• Fixes return types to be fully qualified to enable code analysers to check return types properly.
  Example running phpstan level 9:

  20     Access to offset 0 on an unknown class Kitar\Dynamodb\Model\Illuminate\Database\Eloquent\Collection.                                                                                        
         💡 Learn more at https://phpstan.org/user-guide/discovering-symbols      
  

• Implement PSR coding standards for new objects.
• Fix typo on comment

[kitar]

Thank you for the fix! Looks nice :)