Generate a signature for file uploads

Design files are uploaded directly to storage. This endpoint returns the URL to upload to
(uri) and the form fields the upload must include (form_fields). To upload a file, send a
multipart/form-data POST to uri containing every entry of form_fields, followed by the
file itself in a field named file.

Processing uploaded design files

Design files uploaded with upload_type=pcb are held as pending uploads until you ask
MacroFab to process them:

1. GET /api/v3/pcb/{pcb_id}/{pcb_version}/ingestion/uploads lists the pending uploads.
2. POST /api/v3/pcb/{pcb_id}/{pcb_version}/ingestion/workflow with {"files": [...]} starts
   processing the named uploads, and responds with the workflow's id.
3. GET /api/v3/pcb/{pcb_id}/{pcb_version}/ingestion/workflow/{workflow_id} reports the
   workflow's progress and results.
4. POST /api/v3/pcb/{pcb_id}/{pcb_version}/ingestion/workflow/{workflow_id}/accept starts
   applying the results to the PCB version. Poll
   GET /api/v3/pcb/{pcb_id}/{pcb_version}/ingestion/workflow/{workflow_id}/import until it is
   complete, then call DELETE on the same URL to finish.

The same steps are available interactively from the PCB's page in the MacroFab platform. To
upload only BOM and placement (XYRS) data, use POST /api/v3/pcb/{pcb_id}/{pcb_version}/import
instead.