Add cesium ext structural metadata

[bertt]

Adds support for Cesium extension EXT_structural_metadata

PR at Khronos: https://github.com/KhronosGroup/glTF/pull/2151

Spec: https://github.com/CesiumGS/glTF/tree/proposal-EXT_structural_metadata/extensions/2.0/Vendor/EXT_structural_metadata

Samples: https://github.com/CesiumGS/3d-tiles-samples/tree/main/glTF/EXT_structural_metadata

[vpenades]

I've just updated master branch with a requirement to decorate custom vertex types with a dynamic attribute (see changes), I am still looking for better solutions, but all of them imply breaking changes.

[bertt]

I've just updated master branch with a requirement to decorate custom vertex types with a dynamic attribute (see changes), I am still looking for better solutions, but all of them imply breaking changes.

see https://github.com/vpenades/SharpGLTF/pull/211 , fixes also the github action

[vpenades]

So, if you don't plan to add anything else, it's fine by me to merge, let me know.

[bertt]

Yes it’s finished, ok to merge