Implement Support for RADIUS Authentication

Working on implementing a RADIUS authentication module - guacamole-auth-radius. The basic implementation is completed - with a basic PAP or CHAP RADIUS server, the authentication succeeds and the user is logged in.

I'm running into an issue, though, trying to implement Challenge/Response in RADIUS. I have my RADIUS server configured to talk to LinOTP for MFA/2FA, and RADIUS sends the AccessChallenge package back, asking for the second factor. My issue is in my continual failure to grasp the connection between the servlet side and the AngularJS web application. I've copied the Duo authentication code and tried to morph it into something that will present another box for the RADIUS challenge, but I can't get my controller function to actually fire.

Once that is working, I'd like to support other RADIUS authentication protocols, like EAP-TLS and EAP-TTLS, so there's a little more work to be done, but right now I'm focusing on the basic protocols and the challenge/response.

Will have a repo posted here in a moment for working on this.