Add PMD Apex analyzer

What does this MR do?

This MR adds an analyzer that does ~sast detection on Apex projects projects using the open source pmd tool.

pmd can handle scans for a number of languages, however we have more specialized tools for the primary languages, such as spotbugs to scan Java byte code, so I've scoped this scanner to "apex only" (pmd-apex). If we see a future need to support scans of the other pmd-supported languages (such as VisualForce or Java Server Pages then it could be beneficial to rename/expand this support to simply pmd in the future, wdyt?

TODO

• Resolve outstanding absolute/relative pathing issue in file location
• Update sast CI config to include pmd-apex
• Update gitlab-ee docs to include pmd-apex

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ee/issues/10680

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated for GitLab EE https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/10527
• Documentation created/updated for this project, if necessary
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Job definition updated, if necessary
  • Auto-DevOps template
  • Job definition example
  • CI Templates
• Conforms to the code review guidelines
• Conforms to the Go guidelines
• Security reports checked/validated by reviewer