Refactor API for 3D Tiles 1.1 metadata support

[j9liu]

This PR refactors the plugin API to handle 3D Tiles 1.1 metadata implementation, which allows us to finally upgrade to the newest version of Cesium Native.

TODO:

• [x] Finish writing unit tests for CesiumMetadataValue
• [x] Look over for any missing functions between Unity / Unreal versions
• [x] Get this to compile with clang
• [x] Verify that this works in VR (debug why selecting certain buildings makes the level crash)
• [x] Write unit tests for select classes / methods in:
  • [x] CesiumPropertyTableProperty
  • [x] CesiumFeatureIdAttribute
  • [x] CesiumFeatureIdTexture

Other:

• I didn't add any vector -> string or matrix -> string conversions because if we want to match Unity's string conventions, I'd have to write more code to handle this. I can do this if we want, but wasn't sure if it was a worthy effort.
• Reinterop keeps complaining about the deprecated classes. Is there anything we can do about it?

[j9liu]

I think this PR can finally be reviewed! Though it is a meaty one 😬

[j9liu]

Thanks @kring ! I'll make the changes once you're done with the full review, so that I don't make you lose track of your current review. I also just opened https://github.com/CesiumGS/cesium-unity-samples/pull/65 in the Unity samples with changes based on the new API.

Also, you can test feature ID texture picking with San Francisco Ferry Building, asset 2333904. The script should work without needing to be changed.

[j9liu]

Thanks @kring ! I forgot to mention that I addressed all the comments and this is ready for another look 😅

[kring]

Looks great @j9liu! 🚀