Description of problem:
During the export of a manifest the adapters need an API hook to add files to the signed export.
Per the design at: https://trello.com/c/oVy11kLi we need:
1) Add an optional version # argument to the export REST API.
2) Create an adapter API that takes 3 arguments.
* The directory on the filesystem where any new files should be added
* The consumer object for whom the export is being created.
* The version number passed into the api (1).
3) Create a default adapter that no-ops on this call.
Marking as closed per Candlepin procedures as a fix has been merged or it has been determined to not be an issue.