Improve ETL tooling specifically for writing clean files

[mehalter]

We should make sure the write_clean_file is a relatively sane and generalizable format. At the very least there are a few use cases we should keep in mind:

1. Generating object key prefix/path to easily handle the required structure for Glue crawler/partitioning.
2. Easily give the user the ability to just specify a new file extension without worrying about the actual raw filename.

These could be considered generally useful utilities. So they should go in the glue module if that makes sense or they could live in the more general util module or somewhere similar.