Add offset attribute to box, cylinder and ellipsoid

[hpinkos]

For #6777

Adds offsetAttribute support to BoxGeometry, CylinderGeometry and EllipsoidGeometry

No need to update CHANGES from this because the attribute is private. It will be hooked up to the Entity API level for adding heightReference support to these types

[cesium-concierge]

Thanks for the pull request @hpinkos!

• ✔️ Signed CLA found.
• ❔ CHANGES.md was not updated.
  • If this change updates the public API in any way, please add a bullet point to CHANGES.md.

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

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[bagnell]

Looks good. d6c23ee just has spacing changes.