feat: load Google 3D with short-lived server tokens

Without a browser key, the app asks its server for a short-lived Google
access token (GET /api/google/tiles-token, answering accessToken and
expiresAt) and loads Photorealistic 3D Tiles directly with it as a
header. A tile refused for an expired token gets a renewed one and is
retried once. A server that does not offer tokens changes nothing: the
app continues through ion or the keyless globe as before.

src/app/scene.js

@@ -19,6 +19,7 @@ import { describeError } from './errors.js';
 export async function createApplicationScene({
   requestServices,
   googleApiKey,
+  googleTokens = null,
   cesiumToken,
   credits,
   MapController = MapStackController,
@@ -59,11 +60,12 @@ export async function createApplicationScene({
   registerDataCredits(viewer, credits);
   configureCreditKeyboardAccess(document);
   loaderStatus.textContent =
-    googleApiKey || cesiumToken
+    googleApiKey || googleTokens || cesiumToken
       ? 'Loading Google 3D Tiles...'
       : 'Loading the keyless globe...';
   const photoreal = await loadPhotorealisticTileset(Cesium, {
     googleApiKey,
+    googleTokens,
     cesiumToken,
   });
   const tileset = photoreal.tileset;