Add support to external_id parameter when querying tickets

[ggirotto]

Feature Description
Add support to external_id when listing tickets. Ref: https://developer.zendesk.com/api-reference/ticketing/tickets/tickets/#parameters

Example Use Case
When a ticket is created using external_id, we can easily map our entity to zendesk ticket entity without need to store zendesk id on our entity. But to query the ticket back, we need support to this feature.