feat(units): Add QUDT unit and quantity kind correspondances

[jdacoello]

Adds external reference to unit model QUDT as explained in #828

For example:

the unit kilometer km points to its correspondence https://qudt.org/vocab/unit/KiloM which has lots of extra metadata like translated labels, and conversion formulas.

[erikbosch]

I like it

From a tooling perspective it seems to be a bit superfluous to have both qudt-quantity-kind and quantity here. Maybe make quantityoptional if there is a qudt reference?

[erikbosch]

@sschleemilch @jdacoello - adding the qudt references can be done without changing vss-tools. But what do you think of changing validation logic so that only one of quantity and qudt-quantity-kind are required. I.e. quantity needed only if qudt-quantity-kind is not defined. That way we could limit our file https://github.com/COVESA/vehicle_signal_specification/blob/master/spec/quantities.yaml to only include "custom" quantities.

[sschleemilch]

Fine for me. I can have a look.

Btw I would make it a bit more nested:

qudt:
  unit: ...
  kind: ...

[jdacoello]

Added indentation as suggested

[erikbosch]

MoM:

• D: Same unit can be used for multiple quantities
• D: Makes sense to list at least one, most important to have unit reference
• E: Please review/discussion

[erikbosch]

@jdacoello - could you update the markdown at https://github.com/COVESA/vehicle_signal_specification/blob/master/docs-gen/content/rule_set/data_entry/data_units.md#unit-file-syntax so that it mentions the two new fields as optional fields. Possibly also give an example.

[erikbosch]

MoM:

• D to sync with @sschleemilch and update PR

[erikbosch]

MoM:

• Erik to a new look
• D: Do you want to add checks to vSS-tools?
• E: I can create a ticket and see if @sschleemilch have time to add checks
• Ok to merge after maintainer approval