nordigen_integration: Latest Stable null safe-Release with more Features

[1.5.2] - 17th August, 2021

1. Implemented getEndUserAgreementsUsingUserID({required String endUserID}) functionality to fetch End User Agreements for user identified by a endUserID. Big thanks to @c-louis for getting it done.

2. Changed the Response decode to decode UTF-8 body bytes to handle special ASCII characters. All thanks to @stantemo.

3. Documentation and Unit Tests update to reflect the latest changes.

4. Changed the structure of Unit Tests to make them more readable.

[1.5.0] - 13th June, 2021

1. Steps 5 and 6 of API Documentation working now.

2. Example widget improved. Kudos to Cashtic for running the necessary tests and implementing them!

[1.3.3] - 20th May, 2021

Documentation fix