Add support for CustomShader in a model entity

[ptrgags]

I noticed this morning that ModelVisualizer was not propagating the customShader field to the created Model.

This PR adds a property customShader that lets you pass in a CustomShader instance to the entities.

Example Sandcastle -- applies a custom shader that takes the diffuse color and cycles the color channels RGB -> GBR

@lilleyse could you review?

[cesium-concierge]

Thanks for the pull request @ptrgags!

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Reviewers, don't forget to make sure that:

• Cesium Viewer works.
• Works in 2D/CV.