Download a contact list as CSV

Every contact in import order: phone_e164, name, first_name, language, time_zone, country, external_reference, consent_basis, then one variables.<name> column per custom value to read (a list value as JSON; at most 200 such columns), then a variables column with every custom value as typed JSON. Text a spreadsheet could read as a formula, or that starts with an apostrophe, is prefixed with one apostrophe. The dashboard's Import CSV reads such a file back as the same contacts, custom value types included, consent_basis too. The file streams in import order in chunks of 1,000 contacts, so it has no Content-Length (HTTP/1.1 clients get it chunked, and a download that fails partway fails rather than ending early); at most 50,000 contacts (the list limit), and X-Skysay-Truncated says when a list held more. X-Skysay-Row-Count is the number of contacts when the download started; a contact deleted while it downloads is left out, one added is not included. A pure read, bound to the credential's own workspace.