spike: filetransfers: preupload file processing

[adamdecaf]

I think there's an abstract idea with PayGate to offer file processing after merging, but prior to upload. This encompasses a lot of options for what code is executed, but seems to represent a common pattern. This issue is here to aggregate issues related to this.

• Replace file and upload, create N files (upload N and perhaps original)
• fold/reduce over a file

Examples:

• pipeline: support segmenting files before upload #389 (replace file, two new files created)
• pipeline: support balancing files before upload #388 (replace file, in-place modification)
• filetransfers: FI's require FileHeader modification on upload #354 (replace file, in-place modification)
• Bank requires supplemental file #287 (add file, templated contents, upload both)
• files: support flattening batches before upload #259 (replace file, in-place modification)
• files: support encrypting ACH files before upload #171 (replace file, new filename)