(core) add an access token mechanism to help with attachments in custom widgets

Summary:
With this, a custom widget can render an attachment by doing:
```
const tokenInfo = await grist.docApi.getAccessToken({readOnly: true});
const img = document.getElementById('the_image');
const id = record.C[0];  // get an id of an attachment
const src = `${tokenInfo.baseUrl}/attachments/${id}/download?auth=${tokenInfo.token}`;
img.setAttribute('src', src)
```

The access token expires after a few mins, so if a user right-clicks on an image
to save it, they may get access denied unless they refresh the page. A little awkward,
but s3 pre-authorized links behave similarly and it generally isn't a deal-breaker.

Test Plan: added tests

Reviewers: dsagal

Reviewed By: dsagal

Subscribers: dsagal

Differential Revision: https://phab.getgrist.com/D3488

app/client/components/DocComm.ts

@@ -46,6 +46,7 @@ export class DocComm extends Disposable implements ActiveDocAPI {
   public getAclResources = this._wrapMethod("getAclResources");
   public waitForInitialization = this._wrapMethod("waitForInitialization");
   public getUsersForViewAs = this._wrapMethod("getUsersForViewAs");
+  public getAccessToken = this._wrapMethod("getAccessToken");
 
   public changeUrlIdEmitter = this.autoDispose(new Emitter());