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;
    }

[CLAassistant]

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[andybak]

I've just hit a need for a similar method for Import. Does that belong in the same PR?

Is there anything missing that would help move this PR forward? We're a branch including this PR in production now.

[hybridherbst]

We're looking at the PR again and looks like it will be included with the next version :) We did add a big warning to the Readme that using this hook requires understanding what can go wrong because it's rather easy to shoot yourself in the foot (e.g. skipping export of a few bones in a hierarchy).

[hybridherbst]

Would be great if you take a look at that then and yes, feel free to make a new PR for import functionality!