Support update_url AX method to update profile data

The update_url needs to be sent via the openid client (relying party) to the provider in the fetch request, then:
"If present, the OpenID Provider may re-post the fetch response
message to the specified URL at some time after the initial response
has been sent, using a OpenID Authentication Positive Assertion."
http://openid.net/pipermail/specs/2007-October/002014.html

The update of the values in uni-directional (provider -> relying party) according to the openid specs, the RP should be almost stateless (only caching), so the provider should have always the latest data. We cannot really ensure that the user cannot edit his profile on relying party side, or can we do that?

Things to do on the client side:

• Provide a menu path for the update_url what identifies the user
• Handle the incoming values, update the profile node

Sreg modules are not affected, no update mechanism is available for SReg values.