GLTF: Store extensions in GLTFState and get supported extensions from GLTFDocumentExtension

[aaronfranke]

• Make used extensions be stored in GLTFState. This allows GLTFDocumentExtension classes to add to the used extensions array.
• Add a way to get the GLTF extensions supported by GLTFDocumentExtension. It will check the required extensions during import, and print a message for each unsupported required extension.

These changes were built with #66026 in mind, in fact this PR is a subset of it. These changes are mostly useful with that PR, but technically changing how the list of GLTF extensions is stored, and adding the ability for GLTFDocumentExtension to say which GLTF extensions it supports, is not dependent on refactoring how GLTFDocumentExtension classes are registered with GLTFDocument. So I split out this PR for easier reviewing.

This work was sponsored by The Mirror.

[lyuma]

Everything here is minor. I think it's good overall, and should allow for better handling of extensionsRequired

[fire]

Reviewed.

[akien-mga]

Thanks!