Implement embedded signing in a web app using the Dropbox Sign API

Steps

1. Create a signature request with the embedded `client_id` parameter set to your app's client ID, using `POST /signature_request/create_embedded`.
2. From the response, extract the `signature_id` for each signer (this is distinct from the `signature_request_id`).
3. Fetch the signing URL by calling `GET /embedded/sign_url/{signature_id}` with your API key as HTTP Basic Auth. The response contains a `sign_url` and an `expires_at` timestamp.
4. Open the `sign_url` inside an iFrame on your page using the Dropbox Sign Embedded JS library (`HelloSign.open(signUrl, {clientId: ...})`).
5. Listen for the `sign` event from the library to detect completion and then close the iFrame or redirect the signer.