Converts a params map into {key, value} tuples for Req's
:form_multipart option.
BoldSign's multipart API uses bracket notation for nested objects:
signers[0][name]="Neil"textTagDefinitions[0][type]="Signature"
Files are extracted as {"Files", {binary, opts}} tuples.
Summary
Functions
Splits files from params and returns {file_parts, field_parts} ready
for Req.post!(client, form_multipart: file_parts ++ field_parts).
Flattens a map into form field tuples. Matches the serialization approach used by BoldSign's official Node.js and Python SDKs.