Scala adaptation of Converters and Populators pattern

Original: https://blogs.sap.com/2019/08/29/converters-and-populators-deep-dive/

Contains two implementations of Converter:

1. DefaultConverter is similar to Java, each populator returns copy of target.
2. UnionConverter where target is created by union of populator results using shapeless. Each populator uses own result type.

For usage examples see tests.