Add Rector

[Jean85]

Adding Rector is more than needed; adding it to the 2.0 release helps into adopting as much Rector rules as possible and benefit the most from both the major version bump and the requirement for PHP 8.1

TODO:

• [ ] add Rector to CI
• [ ] apply first run of Rector
• [ ] add more rules
• [ ] migrate to pure DTOs leveraging readonly where possible