arrival_time and departure_time should be optional

According to the spec if a `stop_time` is `Timepoint::Approximate` then `arrival/departure_time` doesn't have to be present (it's up to the consumer to interpolate the times).

If the `timepoint` column isn't present at all then technically I think that at least the first and last `stop_time` of trip must have times but this seems like too much validation at this point on basic transit structs (maybe when Runs are introduced).

Start by locating the transit structs and validation for stop_time, arrival_time, departure_time, and Timepoint::Approximate. Check how optional times are currently rejected; done means approximate stop times can omit arrival and departure times while preserving the existing validation behavior for other cases.

Written by the indexing model from the issue text.