(Offline) Static Code Analysis (working in CI)

[AB-xdev]

As an alternative to Sonar we should try out/introduce Spotbugs, which also works offline and without a secret. See also: https://github.com/xdev-software/standard-maven-template/issues/57

[AB-xdev]

Another option: https://github.com/pmd/pmd

[AB-xdev]

Current state:

• Will not implement SpotBugs as there are serious problems with FP and ignores
• Implemented PMD, will take a bit until it's merged into every nested repo