[Feature] Allow custom windows for aggregations

[lutter]

Description

Right now, aggregations only allow building hourly and daily statistics. It would be good to allow custom windows like 7 days or 6 hours.

The one thorny thing with this is if windows become very short - that will create a lot of aggregates and therefore slow down both indexing and queries. For short windows, we'd need to investigate how much of a problem that might be.