Upgrade EXT_feature_metadata to EXT_structural_metadata

[j9liu]

Closes #407. This PR is a DRAFT that I'm opening to track the remaining tasks on the staging branch.

When complete, this PR will handle metadata based on EXT_structural_metadata, the 3D Tiles 1.1 metadata extension. EXT_feature_metadata should still be parsed, but it will be internally converted to EXT_structural_metadata Batch tables in .b3dm and .pnts tiles will be converted to property tables in EXT_structural_metadata. Feature IDs will be indicated by EXT_mesh_features.

EXT_feature_metadata will no longer be supported.

Progress

• Store batch IDs in EXT_mesh_features
• Add EXT_structural_metadata counterparts to underlying metadata retrieval implementation
• Convert batch tables to EXT_structural_metadata #655
• Remove old metadata implementation files #658
• Rename generated EXT_feature_metadata and EXT_mesh_features files #668
• Rename structural metadata files #669
• Use EXT_structural_metadata in upsampleGltfForRasterOverlays #667
• Create PropertyView base class with normalization handling #713
• Add PropertyAttributeView and PropertyAttributePropertyView. #716
• Miscellaneous EXT_structural_metadata changes #719

Additionally, Cesium for Unreal and Cesium for Unity's metadata systems should be updated to account for differences in the implementation.

Observations

• schemaUri doesn't seem to be handled for EXT_feature_metadata, and EXT_structural_metadata requires either it or schema to be present. Support should be added in a future PR
• Property views on ENUM types are not supported

[kring]

Amazing work @j9liu!