Adds ability to set bottomContainer will not be created by default.

[puxiao]

Cesium.Viewer.ConstructorOptions:

1. Add a bottomContainer option
2. If set to false, the bottomContainer will not be created.

If you don't want to display bottomContainer or CreditDisplay, you can set it like this:

const viewer = new Cesium.Viewer('cesiumContainer', {
    bottomContainer: false
})

I think it will be very convenient.

[cesium-concierge]

Thank you so much for the pull request @puxiao! I noticed this is your first pull request and I wanted to say welcome to the Cesium community!

The Pull Request Guidelines is a handy reference for making sure your PR gets accepted quickly, so make sure to skim that.

• ❌ Missing CONTRIBUTORS.md entry.
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  • Please send in a Contributor License Agreement (CLA) and comment back here to let us know to check this!
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• ❔ CHANGES.md was not updated.
  • If this change updates the public API in any way, please add a bullet point to CHANGES.md.
• ❔ Unit tests were not updated.
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Reviewers, don't forget to make sure that:

• Cesium Viewer works.
• Works in 2D/CV.
• Works (or fails gracefully) in IE11.

[hpinkos]

Thanks for the pull request @puxiao. However, you should not be removing the CreditDisplay from the viewer. Many data providers, including Cesium ion, have attribution requirements for using their data in applications, so hiding this would be a violation of their terms of service.