Add the Countries model

Models to add
- [ ] `Countries` (https://developer.freelancer.com/docs/common/countries)

Please see how models are implemented in the SDK:
https://github.com/joshuadeguzman/freelancer-sdk-flutter/blob/master/lib/models/fl_currency.dart

Use the json_to_dart tool to keep the consistency between models:
https://javiercbk.github.io/json_to_dart/

> NOTE: There are `null` data values from the API. Ensure to add the explicit data types of each property, and don't leave it as `Null` after using the json_to_dart. [This](https://github.com/joshuadeguzman/freelancer-sdk-flutter/blob/master/lib/models/fl_upgrades.dart) is an example of an incomplete model, and will be updated soon as part of the roadmap.

Review lib/models/fl_currency.dart for the SDK's model structure and consult the Countries API documentation linked in the issue. Use json_to_dart as suggested, ensuring nullable API values receive explicit data types rather than Null. Done means a Countries model is added consistently with the other SDK models.

Written by the indexing model from the issue text.