Expose a way to set blend modes with public API

Currently, I don't think there's a way to set a blend mode without knowing something about the private API.

[BlendingState](https://cesiumjs.org/Cesium/Build/Documentation/BlendingState.html) says it's a helper for custom render states, but [renderState](https://cesiumjs.org/Cesium/Build/Documentation/Appearance.html#renderState) is private.

It's come up on the forum [here](https://groups.google.com/d/msg/cesium-dev/40SQ_ao0E3E/g2s1GYYPCQAJ) and [here](https://groups.google.com/d/msg/cesium-dev/3zTt9rZVX08/AwPXs18hBgAJ).

We can either document the current way, or come up with a way to set blend modes, maybe even on the Entity layer?

Start by reviewing the BlendingState and Appearance.renderState documentation, then read the two linked forum discussions for the existing use cases. The issue does not choose between documenting the current approach and designing a public or Entity-level API, so completion criteria must be established before implementation.

Written by the indexing model from the issue text.