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/standalone/application.js

@@ -6,6 +6,7 @@ import { createStandaloneScene } from './scene.js';
 import { createStandaloneControls } from './controls.js';
 import { createStandaloneData } from './data.js';
 import { createStandaloneTools } from './tools.js';
+import { createGoogleTokenSource } from '../maps/googleTokens.js';
 
 // The existing controls and layer catalog contain page-scoped state.
 let constructed = false;
@@ -39,6 +40,9 @@ export function createStandaloneApplication({
       const scene = await createStandaloneScene({
         ...context,
         googleApiKey,
+        // Without a key, Google 3D can still load with tokens from the
+        // app's server when it offers them.
+        googleTokens: googleApiKey ? null : createGoogleTokenSource(),
         cesiumToken,
         loaderStatus,
       });