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: 
See fb6337d which provides best of both worlds - users can choose to use `AssetHashers.SHA1` directly or `@Inject @Named("SHA1") AssetHasher` if they prefer, and anyone can then add their own `AssetHasher` implementation (or override the injected value).