Fix composite tile caching in ModelExperimental

[ptrgags]

Composite (.cmpt) tiles were not handled correctly in ModelExperimental. Since the ResourceCache uses the content URL for cache keys, this is not enough to distinguish multiple models inside a single composite. This PR simply adds a ?compositeIndex=i query parameter to the request, that's enough to distinguish contents in the cache.

Sandcastle for testing -- this includes both the Composite and Composite of Composite models.

As far as testing goes, if you set enableModelExperimental to true in Composite3DTIleContentSpec.js the tests should still pass. Since #10346 will be added soon, it will be simpler to just wait until we set the global flag to true rather than duplicate tests just to remove half of them later.

@j9liu could you review?

[cesium-concierge]

Thanks for the pull request @ptrgags!

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

[j9liu]

@ptrgags this looks good to me, I tried out the 3D Tiles Format sandcastle and composite tileset now displays the intended number of cubes :)

Can you do one quick thing while you're at it -- in the development/3D Tiles Split sandcastle (which is also impacted by this PR), can you change the otherwise in the Sandcastle to catch? I think it's worth fixing that while we're here

[j9liu]

@ptrgags looks good, just waiting for Travis to pass!