Support 3MF triangle paint_color attribute? (for multimaterial 3D printing slicers compatibility)

**Is your feature request related to a problem? Please describe.**

Hi! I'm trying to integrate assimp to OpenSCAD, a CAD tool with a large 3D-printing hobbyist user base, to benefit from its large set of color-aware 3D file formats (see https://github.com/openscad/openscad/pull/5180#issuecomment-2179356486 ).

A core motivation is to be able to write models that can be printed by multimaterial printers (e.g. Prusa + MMU or BambuLab + AMS). The two main slicers in that space ([PrusaSlicer](https://www.prusa3d.com/en/page/prusaslicer_424/) and its fork [BambuStudio](https://bambulab.com/en/download/studio)) use `3MF` with custom attributes.

In particular, when it comes to writing which [material to "paint"](https://help.prusa3d.com/article/multi-material-painting_262620) a triangle with, they use a rather convoluted serialization scheme and the attribute `paint_color` or `slic3rpe:mmu_segmentation` ([more details here](https://github.com/openscad/openscad/issues/5065#issuecomment-2179417639)) to describe how the face is painted.

**Describe the solution you'd like**

So, it would be amazing if assimp were able to write a `.3mf` that can be recognized as native by PrusaSlicer & BambuStudio.

Not sure how that fits into the assimp ethos, but if it's too specific a use case, I was wondering if you'd consider at leaast allowing custom key-value attributes for faces (and maybe custom metadata), so we can generate a model slicers will accept / read face painting info from.

It may be overkill to support the serialization scheme, but at least allowing passing along the serialized values would be great.

**Describe alternatives you've considered**

Alternatively I might just write to a blob, read the zip and surgically edit the model XML to add the attributes, leaving some `matIdx` breadcrumbs to know what to edit.

Orrr fork assimp and alter [D3MFExporter::writeFaces](https://github.com/assimp/assimp/blob/77f7706a97f478c059a88a4384ec49779d7321e9/code/AssetLib/3MF/D3MFExporter.cpp#L337) somehow.

Thanks for the amazing work on assimp & for reading this!

Start with D3MFExporter::writeFaces in code/AssetLib/3MF/D3MFExporter.cpp and review how the exporter currently serializes 3MF face data. Compare that behavior with the referenced PrusaSlicer and BambuStudio paint_color or slic3rpe:mmu_segmentation requirements; done means the written .3mf is recognized natively by those slicers or the requested face attributes can be passed through.

Written by the indexing model from the issue text.