Add ArchUnit architecture checks

[ivamly]

Following the recent migration to JSpecify annotations for nullability constraints, it would be beneficial to add an ArchitectureCheck, similar to the one implemented in Spring Boot using ArchUnit. This would help enforce architectural rules and ensure consistent application of the new annotations across the codebase.

[ivamly]

Hi @bclozel @snicoll !

If you decide to work on this task, is there anything I can do to help? I'd be happy to work on it.

Thanks!

[bclozel]

Thanks for the proposal @ivamly , I'm already working on it.