NEXUS-6323: Plexus cipher reimplementation

Adding a re-implementation of PlexusCipher that uses CryptoHelper.

Issue
https://issues.sonatype.org/browse/NEXUS-6323

CI
NA
btw I've implemented this already on master, the impl here wasn't even following the same practice as the rest of the factory methods in this impl... did you not actually look to see what the others were doing?