Add picocli annotation processor

[remkop]

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)

[remkop]

Work in progress is to add an abstract annotation processor to the picocli-codegen module (no new module). This annotation processor will inspect the source code and build up a list of CommandSpec models from the annotations. Subclasses can override and implement a method to do useful work with these CommandSpec objects.

[remkop]

TODO:

• change relative path to META-INF/picocli-generated/$groupId/$artifactId/...
• if no groupId and artifactId are specified, insert a UUID to ensure a unique location? -> No, that would result in many subdirecties under picocli-generated/...
• cleanup: remove JUL logging (WONT DO)
• polish: add logging via the Messager.printMessage(Diagnostic.Kind) API (at NOTE level?)
  • log user-specified option values, log path where file is generated
  • TBD change groupId and artifactId to group and name? (WONT DO)
  • TBD change groupId and artifactId to a single option: path or coordinates or id or project?
  • log class/method names of @Command-annotated elements in abstract processor (WONT DO)
• warn if ResourceBundle not found? Include ResourceBundle even if not found? fixed bug
• documentation
• Add options disable.reflect.config, disable.proxy.config, disable.resource.config, project
• Other processors may need options like location, relative.path, full.path (but each processor may need a different output location, e.g., autocompletion script, man pages, ...)
• logInfo should prefix simple name instead of full class name
• add isEnabled protected method to abstract proc. Skip processing if disabled. Log "$proc is available but not enabled" when processing done
• test that no files are generated when disabled