(core) custom widget appear as build-in widget

Summary: Added boilerplate code needed to create new wigets in "Add new" menu, that are wrapped around existing custom widgets. More details can be found here: https://grist.quip.com/larhAGRKyl6Z/Custom-widgets-in-Add-Widget-menu

Test Plan: nbowser tests added to verify if item in menu exits, if widget is rendered, and right side menu has widget selection and read access selection hided.

Reviewers: georgegevoian

Reviewed By: georgegevoian

Differential Revision: https://phab.getgrist.com/D3994

app/common/gristUrls.ts

@@ -8,6 +8,8 @@ import {TelemetryLevel} from 'app/common/Telemetry';
 import {getGristConfig} from 'app/common/urlUtils';
 import {Document} from 'app/common/UserAPI';
 import {UIRowId} from 'app/plugin/GristAPI';
+import {IAttachedCustomWidget} from "app/common/widgetTypes";
+import {ThemeAppearance, ThemeAppearanceChecker, ThemeName, ThemeNameChecker} from './ThemePrefs';
 import clone = require('lodash/clone');
 import pickBy = require('lodash/pickBy');
 import {ThemeAppearance, ThemeAppearanceChecker, ThemeName, ThemeNameChecker} from './ThemePrefs';
@@ -666,6 +668,8 @@ export interface GristLoadConfig {
   // TODO: remove once released.
   featureFormulaAssistant?: boolean;
 
+  permittedCustomWidgets?: IAttachedCustomWidget[];
+
   // Used to determine which disclosure links should be provided to user of
   // formula assistance.
   assistantService?: 'OpenAI' | undefined;