Add a possibility to get the count of tickets for some particular period for filtering by dates in API.

Now we use search API to retrieve the number of tickets for some particular period, but it doesn't look like the number is precise enough.

It would be great to have a possibility to get the precise number by sending a request to this endpoint https://developer.zendesk.com/api-reference/ticketing/tickets/tickets/#count-tickets and filter by `created_after` and `created_before`, something like this.