Add an extra date (Occurrence) to a repeating schedule

Add one Shift on one date of the signup's Slot Series: seriesDate (YYYY-MM-DD, inside the Series' range), start / end (HH:MM in the signup's timezone), and optionally label, capacity (default: the Series' own) and location. The new Occurrence is detached from birth: it does not follow the rule, so a later Series edit leaves it alone, and the dates the rule generates are unchanged. On a date the rule already generates it becomes a second Occurrence at the next shiftIndex. A date outside the Series' range, or an end not after start, is 400 bad_request and nothing is written. Adding a date notifies nobody. location is behind a pre-GA rollout for the person an OAuth token acts as (403 forbidden when they are outside it); an API key is not checked.