fix(GLTFCesiumRTCExtension.d.ts): implements THREE.js's GLTFLoaderPlugin

[sguimmara]

This typing is necessary to register this extension on the three.js GLTFLoader.

[gkjohnson]

Thanks! Can you add this on the other GLTF Extensions, as well? (MeshFeaturesExtension, StructuralMetadataExtension)

[sguimmara]

StructuralMetadataExtension

Yes, updated the other files as well :)

While looking at the GLTFLoaderPlugin API, it seems very similar to what you did for the TilesRenderer: an object that exposes optional methods that are called by the invoke plugin mechanism. It might be a good idea to add types for those plugins as well, what do you think ?

Something like that (truncated only for the fetchData method):

export interface TilesRendererPlugin {
        ...
	fetchData?: ( url: RequestInfo, options: RequestInit ) => Promise<Response>;
        ...
}

[gkjohnson]

Thanks! I'll respond to the tiles plugin types proposal in the other PR.