Null Safe and Linted Release

Development of a Null Safe Dart/Flutter Package for Nordigen EU PSD2 AISP Integration with relevant Data Models, proper encapsulation with the exposing of parameters, and succinct documentation.

For more information about the API view Nordigen's Account Information API documentation.

1. ASPSP Data Structure changed to include [String] transaction_total_days.

2. Lint rules applied.

3. Null Safety implemented.

4. Documentation follows null safety conventions now.

5. Lint -> Pedantic for linting.