Assign shift operator

Assign an operator to the shift that machine `machineId` was running at `start`. Use it to record who ran a machine after the shift ended, for example a day later from a planning system. For a live shift at a floor terminal, `POST /v1/realtime/{machine_id}/operator` is the shorter call. The operator takes over at `start` and keeps the machine until the end of the shift, or until a later assignment. If nobody is assigned to the shift yet, the operator covers the whole shift. To record several operators in one shift, send them in chronological order: a `start` before the shift's latest operator change is rejected. Sending the same assignment again is harmless. Operator metrics for the shift are recalculated in the background. Errors: * `400`: no shift matches, or `start` is outside the shift, in the future, or before the shift's latest operator change. * `403`: the shift has production marked as reviewed, which locks it. * `404`: the operator does not exist in your company.