Optional dynamic environment map manager

[techtruth]

Description

Adds option to prevent creation of DynamicEnvironmentMapManager in Cesium3DTileset. This lets cesium run without requiring ownership of a webgl context, resulting in the ability to use cesium 'headless' without a graphical output. I have used this modification to successfully integrate babylonjs as a rendering engine for tiles. https://community.cesium.com/t/accomplished-rendering-with-babylonjs-single-webgl-context/44464

Testing plan

https://github.com/techtruth/cesium_in_babylon/blob/mars/src/cesium_derived/CesiumTilesetDerived.js
Tested above in the demo: https://jamescroney.com/demo/cesiumbabylon
Added unit tests for checking if DynamicEnvironmentMapManager with and without the option enabled.

Author checklist

• I have submitted a Contributor License Agreement
• I have added my name to CONTRIBUTORS.md
• I have updated CHANGES.md with a short summary of my change
• I have added or updated unit tests to ensure consistent code coverage
• I have updated the inline documentation, and included code examples where relevant
• I have performed a self-review of my code

[github-actions]

Thank you for the pull request, @techtruth!

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