Added Cursor Based Pagination support for get tickets

[JinHuangAtZen]

1. Added Cursor Based Pagination support for get tickets method
2. Added Iterator for get tickets, by default, the iterator will use CBP to retrieve tickets.

Example to use the Iterator:

    ops := zendesk.NewPaginationOptions()
it := client.GetTicketsEx(context.Background(), ops)

for it.HasMore() {
	tickets, err := it.GetNext()
	if err == nil {
		for _, ticket := range tickets {
			println(ticket.Subject)
		}
	}
}

[nukosuke]

Thank you!
v0.17.0 has been released.

[cryptomail]

More to come....we will need to iterate across all objects!