Reorganize metadata-related classes under new CesiumMetadata library

[j9liu]

Description

This was done in support of CesiumGS/cesium-unreal#1891, although I acknowledge the connection is not obvious.

CesiumGltf contained various utility classes for accessing metadata from a model's EXT_structural_metadata. However, CesiumGltf intended to be scoped as a lightweight glTF reader, per discussion in #760, and has suffered from some bloat over the years.

Although CesiumGltfContent was introduced for involved interaction/manipulation of glTF models, metadata is more special due to the overlap with 3D Tiles. In particular, the schemas between tilesets and glTF models with EXT_structural_metadata are identical in structure, but have unfortunate duplication between the generated spec classes. This overlap may be resolved with glTF 2.1, but even so, we may need to keep the Cesium3DTiles counterparts to parse older tileset versions.

Thus, this PR implements the suggestion in #760 to create a CesiumMetadata library for all the metadata-related classes.

Issue number or link

N/A

Author checklist

• I have submitted a Contributor License Agreement (only needed once).
• 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.

Testing plan

TODO: Unreal/Unity PRs that implement the changes without breaking.

Reviewer checklist

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