Support neuroglancer precomputed datasets

[normanrz]

Detailed Description

Webknossos supports neuroglancer precomputed datasets. However, it does not support the precomputed meshes yet. 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

[fm3]

duplicate of #6826