NEXUS-7521: simple Asset hashing service

based on Guava's thread-safe hashing functions

https://issues.sonatype.org/browse/NEXUS-7521
http://bamboo.s/browse/NX-OSSF345-1 :white_check_mark: 
Understood, though this does have a limitation of not allowing new impls to come along later (by a plugin for example) w/o code changes to this enum.