Add shouldExportAnimation option on GLTF exporter to filter out animations

[carolhmj]

This add the possibility of the user choosing which animations will be in the exported GLTF.

Related forum issue: https://forum.babylonjs.com/t/how-to-exclude-all-disabled-nodes-from-export/38780/9

[bjsplat]

Please make sure to label your PR with "bug", "new feature" or "breaking change" label(s).
To prevent this PR from going to the changelog marked it with the "skip changelog" label.

[bjsplat]

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