Updates for glTF extension validation

[javagl]

This PR accompanies the PR at CesiumGS/3d-tiles-validator#280

The changes that are done here:

• The GltfUtilities contained a function extractJsonFromGlb that only sliced out the JSON buffer from a glTF 1.0 or glTF 2.0 GLB file. There was not function for obtaining the binary buffer. This has been generalized into a function extractDataFromGlb that returns the JSON buffer and the binary buffer.
• The BinaryPropertyTable originally contained the propertyTable and individual properties that together defined the binary representation of the metadata (independent of whether it was part of a property table or not). The parts that define the binary metadata have now been summarized in a BinaryMetadata structure.
• The handling of ENUM properties and their binary representation had some issues that are supposed to be fixed here. Details are given in Binary representations of enum values are not translated #71

[javagl]

I'll mark this as 'Ready For Review' now. The relevant changes (beyond additional, 'internal exports' that are required for the validator) are summarized in the first comment. This will be part of the change log of the next (minor) release that is supposed to be used for CesiumGS/3d-tiles-validator#280