Create calendar commands

Change one customer's scheduled interaction on a campaign calendar: move it, cancel it, undo an earlier change (naming that receipt's commandId), or schedule a new one. It changes a live schedule, so the user must approve. The Idempotency-Key is the command id: a retry with the same key and body replays the receipt. A refusal is 409 CONFLICT with details.reason: conflict, the revision moved (read again, or take details.current, and retry; the only one that may then succeed); locked, the interaction's state, a paused subscription or an expired undo refuses it; occupied, the customer's month already holds an interaction, live or cancelled; idempotency_key_reused, the command id was spent on another change or caller (use a new key; one the door still holds for another request is IDEMPOTENCY_CONFLICT with that reason). Returns the receipt with the schedule before and after.