Implement a terrain layer selector

[allyoucanmap]

Description

Now that we have a specific terrain layer we could introduce a selector to change the terrain provider of the cesium viewer then store the selected layer in the saved map. The behaviour should be similar to the background selector but the UI could be different. I think this need a mockup.

Acceptance criteria

• [ ] A terrain provider selector component that allow to change terrain provider in the 3D viewer

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