Dropbox Sign Events and Callbacks API

Outbound event / callback surface for Dropbox Sign. The platform POSTs JSON event payloads (wrapped in a `multipart/form-data` `json` field) to a subscriber-configured URL whenever signature requests, templates, and account-level activity change state. Two callback scopes are supported: account callbacks (configured on the authenticated account) and app callbacks (configured per API app and routed by `client_id`). Authenticity is verified via the `event.event_hash` field, computed as HMAC-SHA256 of `event_time + event_type` keyed by the receiving account's API key.