Allow auth plugins to provide custom user signup forms

It would be very useful if authentication plugins that allow self-registration were able to override the built-in signup form. Plugins appear to be able to implement sign-up however they like using the user_signup method, but they're stuck with the existing form which won't always be appropriate.

The main use case I'm thinking of here is for (a custom version of) the Shibboleth plugin to be able to capture Shibboleth attributes as part of the user sign-up. At our institution we're using Shibboleth for all logins, and are hoping eventually to register our server with the UK Federation to enable us to give access to colleagues from other universities. (We were unable to do this before we upgraded to Moodle 2 and were able to switch on "authpreventaccountcreation" since anyone in a trusted university could have potentially created an account, but now we're in a position to do so.) What we need now is a way for people to request Moodle accounts, and I don't see any technical reason why this shouldn't be the self registration page if it was Shibboleth protected.