glTF models should have `asset.version = "2.0"` by default

[javagl]

Sparked by CesiumGS/cesium#13032 :

Apparently, when creating a glTF model and just writing it with the CesiumGltfWriter, it does not contain the asset.version = "2.0" property. First, that's invalid. Second (in case more reasoning is necessary), CesiumJS gets confused by this and can cause hard-do-diagnose follow-up errors.

One could argue that ~"the model should, by default, be completely unititialized" - and that's a valid point. We're not inserting some random Node or Animation just ~"because it is convenient for us". But I think that the asset version is pretty much a baseline.

Given that large parts of the code are auto-generated, and the version field does (of course) not have a 'default' value as of the spec, the question may be where it should be inserted. But there should be some place - maybe around

cesium-native/CesiumGltf/include/CesiumGltf/Model.h

Line 15 in e876c13

Model() = default;

(given that the auto-generated part is ModelSpec anyhow...)