Add authentication hook point

As part of the #1304 (closed), @fdupont and I discussed the evolution of the basic http auth evolution. The code introduced in #1304 (closed) adds a basic credentials storage in the Kea config file. However, there are better alternatives envisaged in the future (keep them in a file, in a DB, perhaps in external system, like PAM, LDAP, RADIUS, etc.). Those could be implemented as hooks.

For this reason, we need a hook point that will:

• expose the credentials provided in a request to be authenticated
• let the hook decide whether the request should be authenticated or not