Add nullness annotations

[NiccoMlt]

Java experience can be greatly improved with annotations that help track nulls, i.e. @Nullable and @NonNull.

In Java history, there are many efforts to do so, like:

• The outdated JSR-305: Annotations for Software Defect Detection
• SpotBugs/FindBugs™ JSR-305 implementation (under edu.umd.cs.findbugs.annotations)
• The Checker Framework annotations
• Jakarta Annotations
• JetBrains Annotations
• ... other? This StackOverflow question and this answer list many of them with details

JSpecify is being created by a group led by Google (with EISOP Team, Google, JetBrains, Meta, Microsoft, Oracle, PMD Team, Sonar, Square, Uber, VMware) with the aim to create a tool-independent nullness annotation standard which is modern enough to replace JSR-305.

It recently received the first stable release with stable APIs to be used, and it seems that it is getting adoption.

[NiccoMlt]

I think an acceptable choice would be to adopt the annotations first with not actual implementations to leverage IDE help during development without introducing anything into the build.

When the adoption in our codebase will be complete, we could start thinking about what implementation introduce into the build to actually check the code.

@dmercuriali @hamadodene what do you think about this?