[uss_qualifier/flight_data_resource] Validate flights, fix flight data, test flight validation

[the-glu]

This PR try to avoid cases likes #1443 by improving flight data validation on loading.

FlightDataResource now validates flights as much as possible, reusing RIDCommonDictionaryEvaluator.

Existing validators are reused with minor tweaks. When injection validation runs without any observation, this is an expected case rather than a real error in the new case, signaled by raising NoObservedFlight.

A few invalid data points have been fixed:

• generate_registration_number in operator_flight_details.py was generating invalid registration numbers.
• kentland/rid.kml had an invalid serial number (original issue). (Has been also fixed in [uss_qualifier] Clean up NetRID automated testing #1446)
• dcdemo.kml had wrong serial numbers.

Test cases have been added for as many situations as possible (missing fields / wrong fields), mainly using ExternalFile which is more 'malleable'.

Notice that the test_invalid_record test was not correct: it was using KML files instead of JSON files, generating a different error, not caught since the test is using Exception. We may want to start using an explicit exception.

Notice that invalid flights may still pass validation, dues to difference between rid versions, but that probably acceptable: during load, we don't know how the flight will be used.

[BenjaminPelletier]

We should at least add a P3 Issue to better encapsulate the validation logic so it can be used directly both here and in the evaluator (instead of strapping on a fake scenario in order to run it) :) But, this does seem to work and addresses a time-sensitive need, so I think this is fine for now.