Add support for Cesium Ion Terrain Provider

[dsuren1]

Description

We should implement support for Cesium Ion resources and assets in MS by recognizing the new Cesium terrain provider, cesium-ion.
This should allow users to configure a Cesium Ion Terrain Provider, with the added flexibility to define the assetId, accessToken & server for the terrain. This new configuration will be used inside the TerrainLayer to implement the new provider

{
  "type": "terrain",
  "visibility": true,
  "provider": "cesium-ion",
  "options": {
    "assetId": 1,
    "accessToken": "",
    "server": undefined  // by default uses https://api.cesium.com
  }
}

Note

• In Cesium Ion, requestWaterMask and requestVertexNormals are not configurable because these settings are managed automatically based on the asset type and data provided by Cesium Ion
• There is no need to configure the ellipsoid explicitly, as the underlying data from Cesium Ion automatically aligns with the WGS84 ellipsoid.
• Since Cesium Ion resources are accessed using an access token, it is assumed that the need for a proxy is unnecessary
• Cesium handles the appropriate attribution and licensing credits when displaying resources from Cesium Ion

Additional Info

Reference ticket - https://github.com/geosolutions-it/support/issues/3946