cas: support setting display name

[tharvik]

Now, the CAS server can return an attribute stating what's the desired displayname, instead of using the username directly.

btw, one nice config block for cas' attributes would be

cas_config:
    server_url: "https://cas-server.com"
    service_url: "https://homeserver.domain.com:8448"
    attributes:
        display_name: name
        required:
            name: value

but that would break existing config.

[erikjohnston]

This looks good to me other than the code styling problem. My ask @richvdh to double check this makes sense from an SSO/CAS PoV

[richvdh]

looks generally good, a couple of comments though

[richvdh]

looks good to me other than a tweak to the changelog wording!

[richvdh]

thank you for the contribution!

fixed