Add "days are always 24 hours" opt-in support to `Timestamp` APIs

Currently, all APIs on `Timestamp` return an error when a `Span` is provided with units bigger than hours. The docs suggest converting the `Timestamp` to a `Zoned` if one wants to use a `Span` with bigger units. But as with the `Span` APIs, perhaps we should permit these APIs to be used with units of weeks/days if the caller explicitly opts into treating them as invariant units.