Provide repackaged version of JavaPoet

[snicoll]

Our AOT processing needs a code generation library and we're quite happy using JavaPoet in Spring Native. It would be quite unusual for the core container to have a direct dependency on a third-party so the plan is to jarjar it in org.springframework.javapoet in the spring-core module.

[snicoll]

We've decided to depend on JavaPoet directly for now with an optional dependency in modules that require it.