Issue 2: policy assertion proposal v1

Attached.
I have crafted the proposal in the form of a separate spec, just to 
demonstrate how a separate spec that combines the callback protocol, 
callback protocol policy assertion, and the wsdl extension for callback 
may look like. Although the attached doc only contains the policy 
assertion and not the protocol or the wsdl extension for callback.

The relevant part is in section 2 of the document and appendix A. I'll 
reproduce the pseudo-schema and the attachment points in WSDL 1.1 below:

<sca:CallbackAssertion [wsp:Optional="true"]? ... >
   ...
</sca:CallbackAssertion>

Policy subject: Endpoint
Can be attached to wsdl:port and/or wsdl:binding

Comments?

-Anish
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