Support loading meshes from remote neuroglancer datasets

[fm3]

#6716 introduces support for neuroglancer datasets. We should also be able to fetch existing meshes. Compare implementation in webknossos-connect

[fm3]

Quoting from #7572

The mesh format is very similar to our mesh format so the frontend should already support it. The backend would need to detect the availability of meshes and adapt the necessary routes. Spec: https://github.com/google/neuroglancer/blob/master/src/datasource/precomputed/meshes.md#multi-resolution-mesh-format