add cesium ext_mesh_features

[bertt]

Adds Cesium EXT_Mesh_Features extension

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

Sample using FeatureId's by custom vertex attribute see https://github.com/CesiumGS/3d-tiles-samples/blob/main/glTF/EXT_mesh_features/FeatureIdAttribute:

Sample using FeatureId's by texture see https://github.com/CesiumGS/3d-tiles-samples/blob/main/glTF/EXT_mesh_features/FeatureIdTexture

[vpenades]

I can't find the code that generates the ext.MeshFeaturesMesh.g.cs file.

Ideally, the PR should include the Build\ SharpGLTF.CodeGen code, so the generator and the generated code can be tracked to the same PR

It is fine to modify the codegen project to either:

• call the method that clones the repository to get the extension from there.
• simply include the schemas as content files (maybe in a LocalSchemas\ directory

[bertt]

Added the schemas (see https://github.com/bertt/SharpGLTF/tree/add_cesium_ext_mesh_features/src/SharpGLTF.Cesium/EXT_mesh_features/schema), next is generating the code.

[vpenades]

I've fixed the code generator, the field access was in the wrong method, and there's no good way of defining a default value, so I completely removed it (which I think it's the best choice)

I also changed the generated class names to something less generic as "FeatureID"

It's odd to me that these cesium extensions are "EXT" extensions, but I think that if they originated from Cesium they should be called something else, I don't know.

Anyway, I also dislike these alternate names, they're provisional until we settle on a better naming convention.