Add 3DTILES_content_conditional

[javagl]

Addresses CesiumGS/3d-tiles#848

This adds the 3DTILES_content_conditional extension specification.

The current state is a draft, largely based on CesiumGS/3d-tiles#834 (commit ad47ad1f7e91b3d4deb9edfc42cb1293b5334306 ), with minor wording tweaks.

There are some TODO_GLTF markers, and many ("normative") details have to be added. (There is an ongoing conversation about this at CesiumGS/3d-tiles#834 )

[weegeekps]

If you merge with the latest 3d-tiles-2.0 branch, I added a REUSE.toml file to the directory that should resolve your REUSE issues.

[javagl]

The failing CI here was only due to the missing REUSE.toml in other directories (i.e. I added one here already), but will check with the next update.

[javagl]

The remaining CI errors should be resolved when #112 is merged.

[javagl]

There currently are a few TODO_GLTF markers in the text.

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The extension originally defined the new content JSON to contain an array of 3D Tiles content objects. It is not clear what the type of these objects should be. While there is a content in 3DTILES_tileset, this does not play the same role as the former content. One could make a very strong case to align this with the new structures, using the externalAsset and file concepts. And actually, it would make sense to let this build upon KHR_node_visibility.

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In the current state, there is no easy mechanism for "wildcards" that allow activating all contents (regardless of their keys). This would be required in order to let this extension easily mimic the 'multiple contents' extension. This question (or any current answers) might become obsolete if this is refactored to be based on KHR_node_visibility.

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The constraints for what the conditional content items can refer to have to be clarified and aligned with the 3DTILES_tileset spec. While they (still, probably) should not be allowed to refer to other conditional contents or external tilesets, concepts like ~"tile formats" are not applicable in 3D Tiles 2.0, and further clarifications may also depend on possible refactorings of the 'conditional content items' concept.