x-callback-url spec, R2 – new “x-cancel” parameter

We’ve gotten feedback that there are use cases for x-callback-url where the user is given the option to “cancel” the requested operation. Previous versions of the spec offered only parameters for success and error, so we added the “x-callback” parameter to the spec to support the case where the user wants to cancel and be returned to the source app. See the spec for details.