[GLTFDocument] extension class. Extends the functionality of the [GLTFDocument] class by allowing you to run arbitrary code at various stages of GLTF import or export. To use, make a new class extending GLTFDocumentExtension, override any methods you need, make an instance of your class, and register it using [method GLTFDocument.register_gltf_document_extension]. [b]Note:[/b] Like GLTFDocument itself, all GLTFDocumentExtension classes must be stateless in order to function properly. If you need to store data, use the [code]set_additional_data[/code] and [code]get_additional_data[/code] methods in [GLTFState] or [GLTFNode]. Returns an array of the GLTF extensions supported by this GLTFDocumentExtension class. This is used to validate if a GLTF file with required extensions can be loaded. This callback is run first. It is used to determine if this GLTFDocumentExtension class should be used for importing a given GLTF file. If [constant OK], the import will use this GLTFDocumentExtension class.