feat: export facemaps

[iuriiiurevich]

This PR adds the feature of exporting face maps as a mesh attribute by enabling "Face Maps" in the "Geometry" options of the exporter.
Face map names are exported to the current node's extras "facemaps".

"nodes" : [
    {
        "extras" : {
            "facemaps" : [
                "FaceMap"
            ]
        },
        "mesh" : 0,
        "name" : "Cube"
    }
],
"meshes" : [
    {
        "name" : "Cube",
        "primitives" : [
            {
                "attributes" : {
                    "POSITION" : 0,
                    "NORMAL" : 1,
                    "TEXCOORD_0" : 2,
                    "_FACEMAPS" : 3
                },
                "indices" : 4
            }
        ]
    }
],

The topic was discussed here: #1232, #1515

Main idea from here: #1232 (comment)
The original code can be found here: https://github.com/canadaduane/glTF-Blender-IO/tree/export-facemaps-and-vertex-groups

Is it a good idea to merge it into the master branch?

[CLAassistant]

All committers have signed the CLA.

[canadaduane]

Thanks for all of your work @scurest and @iuriiiurevich. Is there someone who can authorize it being merged in?

[julienduroure]

A complete refactoring of custom attribute is in progress here: #1676 , and is planned for 3.4
Facemaps will benefit from these refactoring, so this PR will not be merged as-is.