Create an ad hoc (unscheduled) shift for a site via the TrackTik REST API

Steps

1. Look up the target Position ID for the site (and Employee ID if assigning immediately)
2. Authenticate and obtain an OAuth2 access token from the portal's /rest/oauth2/access_token endpoint
3. POST to /shifts with status, billable, payable, timeZone, startDateTime, endDateTime, breakInMinutes, employee, and position fields
4. Set "published": true so the shift is visible to the assigned employee immediately
5. Confirm the new shift appears against the position by re-fetching it with GET /shifts/{id}