Add check for dates which are syntactically correct but invalid

[michaelwood]

We have had a publisher publish "valid" data with an awardDate as 2022-02-29, this causes python's datetime.date.fromisoformat(date) to error with day is out of range for month (which is correct error).

This could be handled at the application (in this case the datastore) that consumes this data to ignore this and replace it with a default or it could be (I think more usefully) raised as validation error.

[robredpath]

ThreeSixtyGiving/standard#283 would allow us to resolve this in schema, I think - although it's described as "format", I believe the "date" format disallows impossible dates, such as 29th Feb in non-leap-years