Add ConfluenceCasAuthenticator for Confluence 4.1+

We are in the process of upgrading our Confluence installation from 3.5.x to 4.2.x. While doing so, we ran into a ClassCastException relating to the stored Principal. (Sometimes when logging in via CAS, other times when logging in via its built-in form.)

After contacting Atlassian support, they mentioned that Confluence 4.1+ no longer supports retrieving or storing the Principal directly from/to the HttpSession. Instead, custom authenticators should use certain inherited methods from ConfluenceAuthenticator.

I simply duplicated the existing ConfluenceCasAuthenticator and made the necessary modifications. I had to bump the Confluence dependency, but the old authenticator shouldn't be affected.