zone: allow `ListZonesContext` to automatically handle pagination
zone: allow `ListZonesContext` to automatically handle pagination

Updates the `ListZonesContext` method to automatically handle the
pagination provided by the API response to return all zones regardless
of the number of entries.

This will fix cloudflare/terraform-provider-cloudflare#790 which uses
this functionality for filtering the data resource.

Fixes cloudflare/terraform-provider-cloudflare#790

cloudflare/cloudflare-go,cloudflare/cloudflare-go