DatagenDriver::export should return a results bag

[sffc]

Currently the function prints a bunch of stuff to the console and returns (). It should return an object with information about the job that completed (still logging the text for CLI users). For example:

1. Resolved options for DeduplicationStrategy
2. Information about the locales that were included or which failed to include. Might need to be per-key.
3. Exporter-specific stats like how many bytes were written (requires adding an associated type to DataExporter)

What needs to be done in 2.0:

1. Changing the signature of the .export function
2. Adding the associated type to DataExporter

What could be done sooner:

1. Make a new .export_with_results function
2. Resolved options and locale inclusion info

CC @robertbastian @zbraniecki

[robertbastian]

I've done this without associated types for now, because we use dyn DataExporter.