Adds support for EXT_mesh_gpu_instancing

[sanjeetsuhag]

This PR adds support for EXT_mesh_gpu_instancing to replace I3DM with glTF for GTModels in 3D Tiles Next.

[sanjeetsuhag]

Merge Changes (3d-tiles-next to instancing)

• TileFormat.h

  • Add createInstancingExtension
  • Rename createFeatureMetadataClasses to createFeatureMetadataExtension

• TileFormat.cpp

  • Resolve needed #includes
  • Choose incoming convertTilesetToJson with urisAtEachLevel argument, set fileExtension to ".glb", use getRelativePathWithNonZeroPaddedLevel
  • Remove redundant ParseJsonAsValue
  • Rename createFeatureMetadataClasses to createFeatureMetadataExtension
  • Keep improved metadata property processing from instancing branch
  • Choose incoming convertTilesetToJson implementation

• CDBTo3DTiles.cpp

  • Keep MATERIALS_SCHEMA_NAME constant
  • Remove duplicate declaration of alignTo8
  • Choose incoming implementation of Converter::convert
  • Keep readBaseMaterialsFile call
  • Keep generateSchema call to write externalSchema

• CDBTilesetBuilder.h

  • Choose incoming functions
  • Remove redundant addElevationAvailability
  • Keep datasetTilesetCollections
  • Keep materials

• CDBTilesetBuilder.cpp

  • Resolve needed #includes
  • Choose incoming addAvailability
  • Choose incoming setBitAtXYLevelMorton
  • Keep addElevationToTileset
  • Keep addGTModelToTilesetCollection but use getRelativePathWithNonZeroPaddedLevel and addAvailability
  • Keep new addAvailability in createB3DMForTileset, use createGLTFForTileset
  • Adds addAvailability to createGLTFForTileset, use createGLTFForTileset

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All tests are passing and output is verified using full versions of sample datasets.