AdUnit Export: Improved exporter encapsulation
 * Introduce "meta-exporter" pattern
   - Automagically finds and delegates exporting to derived specializations
 * Introduce ContentExporter concept to exporter hierarchy
   - Generic encapsulation of common functionality for exporters using content definitions
 * Add renderer CDN uploading to AdUnitTraveltorialExporter

TODO: Get CDN client from factory runtime dependency injection