3.0.0

Feature/null safe (#10)

* Update codebase to null-safety

* Revert tests to dart 2.9 to be able to run tests

* Revert e to Exception

* Revert Object to Exception

* Remove client null check

* Fix formatting

* Add default headers and update FromJson to be non nullable

* Update local var to be final

* Revert is Map<String, dynamic> check

* Fix formatting

* Update version

* Update changelog

* Update github workflows

* Remove empty header parameters

* Rename defaultHeaders to _defaultHeaders

* update null safe branch

* moving null safety to stable

* bump ci

Co-authored-by: David Shen <sdave0921@gmail.com>