feat(storage): Add getDownloadUrl method to the Storage API

etc/firebase-admin.storage.api.md

@@ -8,6 +8,10 @@
 
 import { Agent } from 'http';
 import { Bucket } from '@google-cloud/storage';
+import { File } from '@google-cloud/storage';
+
+// @public
+export function getDownloadUrl(file: File): Promise<string>;
 
 // Warning: (ae-forgotten-export) The symbol "App" needs to be exported by the entry point index.d.ts
 //