Make `GraalVmProcessor` Arguments Optional

[ppkarwasz]

In response to feedback from third-party Log4j plugin developers (see #3755), we should make the -log4j.graalvm.groupId and -Alog4j.graalvm.artifactId processor arguments optional.

Proposed Approach

We can adopt a strategy similar to picocli’s annotation processor. Specifically:

• If the -Alog4j.graalvm.groupId and -Alog4j.graalvm.artifactId arguments are provided, generated files should be written to:

  META-INF/native-image/log4j-generated/<groupId>/<artifactId>
  

• If the -Alog4j.graalvm.groupId and -Alog4j.graalvm.artifactId arguments are not provided, files should be stored under:

  META-INF/native-image/log4j-generated
  

  Additionally a warning should be issued about the lack of those arguments.