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: 
@mcculls unsure, and I don't think its a big issue, was just wondering why you went this direction.

I think this is likely fine, though probably not a model I'd recommend to follow generally due to limited ability to dynamically add new behavior.

An example might be for someone who can't use SHA1 and needs a SHA2.

And I'd expect a generic hashing service to be able to cope with requirements like this, and not force folks to dance around it with custom code, vs say configuration to say use "sha2" instead. etc.