Specifies a fingerprint for cancelPreview() requests. A fingerprint is a randomly generated value that must be provided in cancelPreview() requests to perform optimistic locking. This ensures optimistic concurrency so that the deployment does not have conflicting requests (e.g. if someone attempts to make a new update request while another user attempts to cancel a preview, this would prevent one of the requests). The fingerprint is initially generated by Deployment Manager and changes after every request to modify a deployment. To get the latest fingerprint value, perform a get() request on the deployment.