Support an export ID option

https://redmine.teklia.com/issues/7497

When starting a CLI export from the frontend, users will have to either select an existing SQLite export or create a new one. To support selecting an existing export, all export workers should have an optional export_id option in the user configuration, which will be set to the UUID of the CorpusExport selected by the user. When this is set, the worker should download this specific export instead of the latest one.