Add "ShouldNodeExport" hook

[andybak]

Example usage:

    public override bool ShouldNodeExport(GLTFSceneExporter exporter, GLTFRoot gltfRoot, Transform transform)
    {
        Type[] excludedTypes =
        {
            typeof(FooBar),
            typeof(BarBaz)
        };
        bool hasExcludedComponent = excludedTypes.Any(t => transform.GetComponent(t) != null);
        return !hasExcludedComponent;
    }

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