PATCH fleet/drivers/.id: Adds support for assigning start/end-of-day driver-app workflows.

The PATCH fleet/drivers/.id endpoint has some new features! startOfDayWorkflowId and endOfDayWorkflowId can now be set via the driver API to assign driver-app workflows. Sending null or an empty string for startOfDayWorkflowId clears the start-of-day workflow; clearing endOfDayWorkflowId is not yet supported. Gated by the mobile-driver-workflow-list-api feature config.