Add access to gltf from entity.

Entity has not any direct relation to loaded (or cashed) gltf model. So if I add my custom extension to gltf, I can`t normally read it in case I am using EntityApi. But primitives has this relation. It would be great to add this field to entity. Moreover i wanna edit this gltf. I know this case is very specific, but you made wrapper over PrimitiveApi, and cut off direct access to this Api.

Start by comparing EntityApi with PrimitiveApi to understand how loaded or cached glTF models are exposed. Define the intended EntityApi access and editing behavior for custom glTF extensions, then verify that the resulting API preserves the existing PrimitiveApi relationship.

Written by the indexing model from the issue text.