Creates or updates multiple shifts in bulk. This is an asynchronous API that returns immediately with an operation ID in the response metadata.operation.id field. Shifts without an ID will be created, while shifts with an ID will be updated. Use the operation ID to poll the /operations/v2/operations/{id} endpoint to check completion status. The operation processes upserts in the background. External IDs can be provided to track shifts from external systems. Optional override settings can bypass normal validation rules.