Add Authenticator interface

[Mihael Schmidt]

The Authenticator interface gets the retrieved token from the AuthenticationProvider and passes it to the Authenticator implementation. This needs to be done before the HTTP message is routed to the endpoint.

API proposal: void droplet_auth_authenticate(pointer token)

The Authenticator interface does not send the return message for an unauthenticated request as the return HTTP message may need implementation specific information (like WWW-Authenticate: Basic realm="User Visible Realm" HTTP header).