Divide initializr-generator-spring into two modules, one for its API and one for its @Configuration classes

[wilkinsona]

See #1283 for background.

The general idea is to split the Spring module into two pieces. The first would provide all of the customisers and contributors and the second would provide the configuration classes that creates the default arrangement. Anyone wanting complete control could then use the first module on its own. This is somewhat analogous to Spring Boot's spring-boot-actuator and spring-boot-actuator-autoconfigure modules.