Spring Validation

I'm using this project to use as a demo/reference project on how to improve on Spring validation of incoming data. The idea is to help me get away from Primitive Obsession, and also improve how data can be validated.

Improvements to make

1. Add more tests/examples for ObjectWithHeaders and RequestEntity.
2. Write all the same code in Kotlin, add to this project. This is why the current controller method paths all start with /java.