Add support for 3DTILES_content_conditional in serializer / deserializer

[lilleyse]

Description

Adds support for 3DTILES_content_conditional in Cesium3DTiles, Cesium3DTilesReader, and Cesium3DTilesWriter. This is used by an internal pipeline generating time-dynamic 3D Tiles.

Note: this is still a draft extension, but should be safe to merge as not too many changes are expected.

Issue number or link

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• I have done a full self-review of my code.
• I have updated CHANGES.md with a short summary of my change (for user-facing changes).
• I have added or updated unit tests to ensure consistent code coverage as necessary.
• I have updated the documentation as necessary.

Remaining Tasks

Testing plan

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[j9liu]

Thanks @lilleyse! Just a few minor comments to address before merge.

[lilleyse]

@j9liu ready for another review!

[j9liu]

Thanks @lilleyse!