Added PMD Metrics Parser

[maxwai]

Add new Parser for the metrics report format being added in PMD pmd/pmd#5166

Testing done

Parser was tested using Unit Tests and also tested with https://github.com/uhafner/warnings-ng-plugin-devenv using a modified version of the coverage-plugin that will be merge on a later date.

Submitter checklist

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[uhafner]

Looks already very promising! Just some minor comments and some questions.