- Turn `ImageReference` into its own type so it is validated early on in request submission time, and we don't end up with malformed IDs, etc.
- Add in support for shared image galleries since that was easy enough to add while I'm doing this.
- Explicitly document which image sources are permitted and how to reference them with resource IDs.
This addresses/closes #1464 for the C# port. Also fixes#2927 which was recently reported.