chore: PMD rule for @Nonnull / @Nullable on fields of public types

[newtork]

Follow-up #368

Context: mapping `int priority` to `severity`

[MatKuhr]

I'm a bit unsure about this one. Those are a lot of warnings, and adding @Nonnull everywhere is kinda annoying / code bloat.

I guess the unwritten rule should be: If it's nullable always annotate it as such. Otherwise it will be assumed to be nonnull.
Unfortunately we can't make a PMD rule for that I guess. Would have to be some static code style analysis that goes through the AST and tries to figure out if null can potentially be assigned to a field not annotated as such..

[MatKuhr]

I'd argue the issue is also relevant for non-public classes..

[CharlesDuboisSAP]

There is an IntelliJ warning Suggest @Nullable annotation that we could enable and that catches the NPE in #368

[CharlesDuboisSAP]

lombok says adding @Nonnull everywhere is is massive performance penalty and waste of resources.
They also recommend using an IDE level reminder for this use case.