Unique Object ID

Hi all, as discussed on [Babylon Exporter GitHub ](https://github.com/BabylonJS/Exporters/pull/691)we have a situation where many objects across differents GLTF files are sharing animations.
Also, we are working with files that contain more than 5000 nodes and artists are struggling with a pipeline where the moment a single keyframe is changed the entire scene must be re-exported.

We implemented this solution, proposed by @Selmar on #1448. Adding a unique identifier to every node and animation channels target.
This way we can eventually split the animation from the model and, on the engine side, we can blend data coming from different GLTF files.

As we would like to share this extension to BabylonExporter we are discussing on the way the specification should be implemented
We wonder if the uniqueness of the object should be an isolated extension, were eventually, some other extensions can be built on top of that.

What do you think about this?
should we have a UniqueIdentifierExtension that will end up with something like this?

``` "nodes": [
{
"skin": 0,
"mesh": 0,
"translation": [
0.0,
-0.0287759788,
-0.0076726065
],
"name": "L_Shoulder_JNT",
"extensions": {
"ASOBO_unique_id": {
"id": "shoulder"
}
}
},```

Start by reviewing Babylon Exporter PR #691 and the discussion in issue #1448, then examine the proposed node and animation-channel identifier shape in this issue. Define whether the identifier belongs in an isolated extension and document the agreed specification.

Written by the indexing model from the issue text.