(core) Extending Google Drive integration scope

Summary:
New environmental variable GOOGLE_DRIVE_SCOPE that modifies the scope
requested for Google Drive integration.
For prod it has value https://www.googleapis.com/auth/drive.file which leaves
current behavior (Grist is allowed only to access public files and for private
files - it fallbacks to Picker).
For staging it has value https://www.googleapis.com/auth/drive.readonly which
allows Grist to access all private files, and fallbacks to Picker only when the file is
neither public nor private).
Default value is https://www.googleapis.com/auth/drive.file

Test Plan: manual and existing tests

Reviewers: dsagal

Reviewed By: dsagal

Subscribers: dsagal

Differential Revision: https://phab.getgrist.com/D3038

app/common/uploads.ts

@@ -42,3 +42,12 @@ export interface FileUploadResult {
  * the page's <base> will be respected.
  */
 export const UPLOAD_URL_PATH = 'uploads';
+
+/**
+ * Additional options for fetching external resources.
+ */
+export interface FetchUrlOptions {
+  googleAuthorizationCode?: string;   // The authorization code received from Google Auth Service.
+  fileName?: string;                  // The filename for external resource.
+  headers?: {[key: string]: string};  // Additional headers to use when accessing external resource.
+}