Add callable to ExportJob.data_file so that storage option is configurable

[matthewhegarty]

Use case: When running on AWS I have s3 storage enabled for other file persistence. However, when exporting I want the file data stored in the db row (default case). I would like to have the flexibility to set this at runtime.

We can use the storage callable to define a hook which we can override. Perhaps the hook can be in settings so that I can define in my implementation.