Read a ready export's contents

The export's text, a slice at a time, under the same API key, for a caller that cannot fetch `download_url` (an agent sandbox that only reaches allow-listed hosts, say). Read from `offset` 0 and pass `next_offset` back until it is null; the slices join into the exact file.

curl -X GET "https://example.com/v1/exports/string/content"

{  "id": "string",  "object": "export_content",  "format": "csv",  "offset": 0,  "next_offset": 0,  "total_bytes": 0,  "content": "string"}