Submits a defense against a chargeback, including supporting documentation.

A chargeback can only be defended while its status is pending. If the due
date has passed, the request returns an error. Defending an already-defended
chargeback returns 204 (idempotent). Only one defense can be created per
chargeback request.

The document.document_base64 payload must be 15 MB or less. Supported types
are PDF, JPG/JPEG, PNG, and GIF; the type is inferred from the file's bytes,
not the filename.