Build Assimp with Draco support

[alexrp]

Draco is a form of graphics compression for formats like glTF.

To do this, we'd like to avoid having to ship an extra shared library in the .Native NuGet package, and instead link Draco statically into the Assimp shared library. I've submitted an upstream PR to enable us to do this: assimp/assimp#5535

[alexrp]

Upstream PR merged, now we just wait for a release tag that includes it.