Update `CesiumWebMapTileServiceRasterOverlay` to match Unity implementation

[j9liu]

This was a followup to https://github.com/CesiumGS/cesium-unity/pull/446 that I didn't get a chance to do until now :') From changelog:

• Renamed the following properties on CesiumWebMapTileServiceRasterOverlay:
  • Url -> BaseUrl
  • West -> RectangleWest
  • South -> RectangleSouth
  • East -> RectangleEast
  • North -> RectangleNorth
• UseWebMercatorProjection has been deprecated on CesiumWebMapTileServiceRasterOverlay. Instead, use the Projection enum property to specify between Geographic or WebMercator projection.
• Added TileWidth and TileHeight properties to CesiumWebMapTileServiceRasterOverlay.
• Fixed a bug where CesiumTileMapServiceRasterOverlay, CesiumWebMapServiceRasterOverlay, and CesiumWebMapTileServiceRasterOverlay would attempt to load empty URLs.

You can test that these changes upgrade seamlessly by:

1. BEFORE checking out this branch, add a WMTS overlay to Cesium World Terrain. Put some non-default values in the properties.
2. Check out + build this branch, and reopen the level.
3. Confirm that the property values were preserved.