NEXUS-6323: Plexus cipher reimplementation

Adding a re-implementation of PlexusCipher that uses CryptoHelper.

Issue
https://issues.sonatype.org/browse/NEXUS-6323

CI
NA
I'm probably missing something, but it seems this abstract class has two identical subclasses that don't add any functionality - could that be collapsed? (Or is the class name used as a lookup for something elsewhere?)