Explore rewriting `QueryUtils.requiresOuterJoin` and `toExpressionRecursively` to use `PersistentEntity` meta-model

We have accumulated quite a bit of code in both utility methods that operate on the level of JPA's `Metamodel`. For using the meta-model in JPQL queries, we've introduced `JpqlUtils` that basically duplicates functionality. We should explore whether we could rewrite this functionality based on our mapping-metadata by considering the JPA meta-model as metadata source where required.