files: support flattening batches before upload

[adamdecaf]

Our ACH library has support for "flattening batches" which means to merge EntryDetail records with the same BatchHeader together. This helps cut down costs and offer smaller files.

The library support was added recently along with an HTTP endpoint if we wanted, but paygate should be able to use the library method directly.

This should be hidden behind a config and I'm not sure if it should be enabled by default or not.