[#215] Add hook to allow customising token challenge response

[funkybob]

Passes details to a method to create the response used when a code is required.

Allows providing any extra context the application requires.

[funkybob]

This could possibly be made a bit leaner if the various parts were stored on the view class instance along the way -- mfa_method, dispatch response, etc.

Then they wouldn't need to be passed into the method.