Azure overlay

[david-lively]

Description

This PR adds support for Azure maps. This includes a new CesiumAzureMapsRasterOverlay component, supporting managed classes, and the native implementation.

This depends on CesiumGS/cesium-native#1264 so merge that first.

Author checklist

• I have submitted a Contributor License Agreement (only needed once).
• I have done a full self-review of my code.
• I have updated CHANGES.md with a short summary of my change (for user-facing changes).
• I have added or updated unit tests to ensure consistent code coverage as necessary.
• I have updated the documentation as necessary.

Testing plan

1. Open the 01_CesiumWorld scene in the cesium-unity-samples project.
2. Add a CesiumAzureMapsRasterOverlay component to the Cesium World Terrain GameObject
3. Configure the component with a valid Azure subscription key.
4. Observe the active overlay in the viewport.

[kring]

A few comments below, but this looks good and works well!

[kring]

Thanks @david-lively!