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 had this change made on branch 5 days ago, but I did it only as I suspected that it belongs to CryptoHelper (and probably missed the part you ask about). Thanks for doing it on the master, will merge it.