Add illegal-transitive-dependency-check enforcer plugin

This adds https://github.com/ImmobilienScout24/illegal-transitive-dependency-check which will check that code uses dependencies that have been explicitly added rather than using transitive dependencies.

It has been defaulted to failing the build if it finds a problem. If `-Dillegaltransitivereportonly=true` is set then it will create report files (e.g. itd-xxxxx-<version>.txt) in the target directory for each module.
Once the plugin is fixed, I'll upgrade it and remove the property.