Miscellaneous cleanup and code generator changes (part 2)

[lilleyse]

Follow up to #409

@IanLilleyT and I have been battle testing the code generator with a variety of glTF and 3D Tiles extensions. The changes have been extracted out here so that future PRs will be easier to review.

• Handle extensions that have schema files with the same name. This is to support EXT_feature_metadata and EXT_mesh_features simultaneously. The main thing here was adding more overrides to glTF.json. This technique relies on the schema titles to be unambiguous. In the future we might need a mapping from <extension name, title> to override name.
• Allow for multiple schemas to be processed together. This is for generating classes for tileset.schema.json and subtree.schema.json which are independent schemas but belong to the same project Cesium3DTiles.
• Support extensions of extensions
• Better handling of schema inheritance in the generated writers
• Support different enum variations, like those seen in EXT_meshopt_compression and KHR_lights_punctual
• Added autogenerated extension registration to the generated writers

Other changes

• Renamed GltfReader::readModel, ModelReaderResult, and ReadModelOptions to GltfReader::readGltf, GltfReaderResult, and GltfReaderOptions. This is for better symmetry with the GltfWriter API in Update GltfWriter to use autogenerated files #416 and the TilesetReader and TilesetWriter APIs.
• Added TilesetWriterOptions for writing tilesets. The only option right now is prettyPrint, which defaults to false.
• Added CESIUM_TESTS_ENABLED option to the build system
• Renamed some test files:
  • TestReader to TestTilesetReader
  • TestReader to TestGltfReader
  • TestModelEXT_feature_metadata to TestExtensionModelExtFeatureMetadata
• Updated async++ so that it works in C++ 20: Replaced std::result_of with std::invoke_result for c++17 or greater Amanieu/asyncplusplus#47