Annotation processor to build a CommandSpec model from the picocli annotations in the source code at compile time.

This would make it possible to build tools for picocli-based applications that do more interesting work during the build:

• generate auto-completion scripts as part of the build
• generate man-page like documentation as part of the build (Linux friendly manpage generation #459)
• generate configuration as part of the build (Using picocli with GraalVM's native-image tool #410, Add module picocli-codegen #499, Annotation processor that generates reflect-config.json during build #699, Annotation processor that generates resource-config.json during build #703, Annotation processor that generates proxy-config.json during build #704)
• generate java source code or instrumented byte code so picocli can work without runtime reflection (Running annotation-based picocli applications without runtime reflection #539)