datetime handling - some invalid datetimes accepted, some invalid ones return incorrect status code

[philvarner]

The server accepts numerous datetime values that are not valid RFC 3339 datetimes. This is because the server does minimal validation and passes the datetime values to ES. There should be RFC 3339 validation so users don't create non-portable datetime queries (e.g., that fail on other STAC API servers).

luxon has a method DateTime#fromISO that gets us part the way there. These should also be run through the regex outlined here https://github.com/radiantearth/stac-api-spec/blob/master/implementation.md#datetime-parameter-handling

warnings:

• Search with datetime=1985-04-12 returned status code 200 instead of 400
• Search with datetime=1937-01-01T12:00:27.87+0100 returned status code 200 instead of 400
• Search with datetime=1985-12-12T23:20:50.52 returned status code 200 instead of 400
• Search with datetime=1985-04-12T23:20:50.Z returned status code 200 instead of 400
• Search with datetime=1985-04-12T23:20:50,Z returned status code 200 instead of 400
• Search with datetime=1986-04-12T23:20:50.52Z/1985-04-12T23:20:50.52Z returned status code 200 instead of 400
• Search with datetime=1985-04-12T23:20:50,52Z returned status code 200 instead of 400

errors:

• Search with datetime=1985-04-12t23:20:50.000z returned status code 500
• Search with datetime=37-01-01T12:00:27.87Z returned status code 500 instead of 400
• Search with datetime=21985-12-12T23:20:50.52Z returned status code 500 instead of 400
• Search with datetime=1985-13-12T23:20:50.52Z returned status code 500 instead of 400
• Search with datetime=1985-12-32T23:20:50.52Z returned status code 500 instead of 400
• Search with datetime=1985-12-01T25:20:50.52Z returned status code 500 instead of 400
• Search with datetime=1985-12-01T00:60:50.52Z returned status code 500 instead of 400
• Search with datetime=1985-12-01T00:06:61.52Z returned status code 500 instead of 400
• Search with datetime=1990-12-31T23:59:61Z returned status code 500 instead of 400