API function on Python Viewer to inject textures independently from mjModel

### The feature, motivation and pitch

The ability to add a new geom to the user_scn in the Python viewer is quite powerful in building and updating all sort of visualization needs -- from annotation to sensor data viz. We have been using this functionality extensively.

For a newly added geom, we also want to add and use a new user texture & material. This feature is currently not supported. This is a missed opportunity as it can unlock a wide variety of cool visualization features on top on simulation which should be lean and thin.

After discussion with @yuvaltassa and @saran-t, it was proposed that we use "a new API function to inject textures and materials independently from mjModel".

Starting this request to track the developments.

### Alternatives

_No response_

### Additional context

_No response_