Instead of this project's current iteration, which is a sample, we can
replace it with a library that has a Spring Boot project utilising it.

This means we need to:

- move the Spring Boot code to the test source set
- update the package structure (including coordinates) to better match
  where it would be published)
- update the version to mirror the javax api
- rename `ControllerTest` to `ControllerIntegrationTest`
- add missing integration test for logging from `BodyReadFilter`, to
  ensure it's hooked in correctly