[NEXUS-5741] Enable file content validation for P2 repositories

https://issues.sonatype.org/browse/NEXUS-5741
https://bamboo.zion.sonatype.com/browse/NX-OSSF49

Added generic file validator on P2 repositories. To do so, I extracted generic validation (based on mime utils) from maven specific content validator and reuse it in a p2 specific one.

Along the way I also replaced plexus with jsr330 for validators and friends