Investigate registering the target type of reading converters as simple types

[schauder]

This is triggered by https://github.com/spring-projects/spring-data-relational/issues/1194

The registration of simple types from reading converters was explicitly removed as part of https://github.com/spring-projects/spring-data-mongodb/issues/1203

If we reintroduce it now, we need to make sure all modules work properly with that change.