Some RPs use callback arguments to avoid using sessions to store 
information that they want to have when the authentication assertion comes 
back.  To prevent in-transit or OP tampering with these callback values, we 
should include them in the signed token the RP already creates.

As part of this, callback arguments would be moved from the plain-text 
return_to argument to the token argument, not only so it can be signed but 
so the values can only be fetched using the new 
IAuthenticationResponse.GetCallbackArgument methods, which will refuse to 
return callback arguments that do not have a valid signature.