Add a tool to manipulate plugin descriptors (#139)

The tool is based on picocli and supports the following commands:

* `toJson`: converts a `Log4j2Plugins.dat` to a JSON representation.
* `fromJson`: converts the JSON representation of a plugin descriptor to its `Log4j2Plugins.dat` form.
* `filterReflectConfig`: filters a GraalVM `reflect-config.json` file by removing the classes that are not contained in a `Log4j2Plugins.json` file.
17 files changed