NEXUS-5961: plexusplugin plugin

A trivial plugin using Plexus components used for testing Plexus DI still works. IT added to boot Nx with this new plugin up and test drive it (verify it lives okay).


Issue:
https://issues.sonatype.org/browse/NEXUS-5961

CI:
https://bamboo.zion.sonatype.com/browse/NX-OSSF78