Get a list of contacts.

Pagination. Every response carries paginator.nextCursor. To get the next page, send it
back as cursor together with the same filters; it is null on the last page. Following
nextCursor reaches every contact, however many there are.

page is still accepted, but it only reaches the first 10,000 contacts: a page past that
returns no contacts. paginator.totalItemsCount is capped at 10,000 for the same reason.
Use cursor to go further.

Order. Without a search filter (fsf_query, name, email or phone_number)
contacts are returned oldest first. With a search filter they are ordered by name; there,
the nextCursor of a page fetched without a cursor may return again the contacts already
returned that share a name with the last contact of that page (up to a page of them when a
search matches many contacts of the same name), but it never skips one. Deduplicate on id
and channel_id when you need each contact once.