Enrich testdata builder with add methods for collection types

[maybeec]

For fields of type collection, we currently generate a generic method, which enables setting the whole collection while building the pojo. It would be nice to also provide an add-Method, which enables adding object by object rather then setting the whole collection. This makes the code using the builder much more readible.

(target branch: master)

[github-actions[bot]]

Stale topic. Please negotiate closing or discussing the relevance of this ticket.