first step for RRL support: introduced libauth and added RRLKey class

This is a first step toward a complete support for RRL.

I've introduced a separate library that will eventually be used from bundy-auth but could also be used as a separate library module for authoritative DNS services.

The added class is quite straightforward port from the BIND 9 implementation, just C++-fied  (and therefore safer) with lots of documentation and unit tests.