Add support for idempotency keys when creating tickets

[aleksei-averchenko-wise]

This change implements support for Zendesk's idempotency key
feature when creating tickets. I happen to need to use this feature myself :)

What's Changed

• Clients can now set an idempotency key on Ticket objects before creation
• The client automatically sends the Idempotency-Key header to the API
• The response is parsed to determine if this was a new ticket creation or a duplicate request
• New fields on Ticket:
  • getIdempotencyKey() / setIdempotencyKey(String) - the idempotency key
  • getIsIdempotencyHit() / setIsIdempotencyHit(Boolean) - whether this was a duplicate request

See the usage example in the README.

Caveat

The current implementation doesn't properly update idempotencyKey and isIdempotencyHit when creating a ticket via queueCreateTicketAsync. I don't know what the best approach would be here, any comments are welcome.