Update unit checks after adding qudt

In https://github.com/COVESA/vehicle_signal_specification/pull/900 we extend the unit with qudt. We could think of extending synatx check, like if qudt is used verify that it contains unit and kind

We also talked about that requesting a "quantity" is maybe superfluous if you specify qudt. So we could think of chaning syntax so it is sufficient to have one of them (but ok to have both if needed)

Fyi @sschleemilch

Start with PR #900 and the existing unit and syntax-check implementation, then trace how qudt, unit, kind, and quantity are represented and validated. Done means the agreed combinations are enforced and tests cover the new validation rules.

Written by the indexing model from the issue text.