Draft changes for External Assets and Unified File References in glTF 2.1

[weegeekps]

This PR contains the draft changes for External Assets and Unified File References in glTF 2.1. This PR currently points towards the draft-2.1 branch and should be safe to merge to that branch.

Relevant Issues:

• [glTF 2.1] External Assets #2586
• [glTF 2.1] Unified File References #2590

Summary from Copilot:

New external asset referencing system:

• Added top-level files and externalAssets arrays to the glTF schema, enabling assets to reference external files and compose complex scenes from multiple glTF files. (glTF.schema.json, file.schema.json, externalAsset.schema.json, [1] [2] [3]
• Nodes can now reference external assets via an externalAsset property, which cannot be used together with mesh. (node.schema.json, specification/2.1/schema/node.schema.jsonL40-R44)
• Introduced the concept of file aliases, allowing URIs within external assets to be remapped for efficient resource sharing or overriding. (file.schema.json, file.alias.schema.json, [1] [2]

Specification and documentation updates:

• Updated the specification to describe the new unified file reference system, external asset referencing, and aliasing mechanism, including normative requirements and implementation notes. (Specification.adoc, [1] [2]
• Added a new appendix with diagrams and example JSON to illustrate the expected scene graph and asset hierarchy when using external assets. (Specification.adoc, specification/2.1/Specification.adocR3516-R3644)