AMQP CBS draft additions

Based on Friday's discussion, I've expanded the AMQP CBS spec with more context around AuthN/Z and also a discussion of the token provider model without getting too deep into the specifics.
Doc is attached, new sections are highlighted.
Attachment: amqp-cbs-v1.0-wd07.doc Description: amqp-cbs-v1.0-wd07.doc