TimeIntervalRounding is not optimised for fixed-offset timezones #46771
Description
In #46670 we added a fast path for fixed-offset timezones to TimeUnitRounding, making it play nicely with #30534. However today there is no such fast path through TimeIntervalRounding. Kibana sometimes performs an aggregation using 3h buckets (e.g. when searching a week's worth of data in the Discover tab) and I think we could make such an aggregation quicker in cases where the timezone offset is fixed.