(core) Exposing custom widgets on the UI

Summary:
Exposing custom widgets as a dropdown menu in custom section configuration panel.

Adding new environmental variable GRIST_WIDGET_LIST_URL that points to a
json file with an array of available widgets. When not present, custom widget menu is
hidden, exposing only Custom URL option.

Available widget list can be fetched from:
https://github.com/gristlabs/grist-widget/releases/download/latest/manifest.json

Test Plan: New tests, and updated old ones.

Reviewers: paulfitz, dsagal

Reviewed By: paulfitz

Differential Revision: https://phab.getgrist.com/D3127

app/common/CustomWidget.ts

@@ -0,0 +1,39 @@
+/**
+ * Custom widget manifest definition.
+ */
+export interface ICustomWidget {
+  /**
+   * Widget friendly name, used on the UI.
+   */
+  name: string;
+  /**
+   * Widget unique id, probably in npm package format @gristlabs/custom-widget-name.
+   */
+  widgetId: string;
+  /**
+   * Custom widget main page URL.
+   */
+  url: string;
+  /**
+   * Optional desired access level.
+   */
+  accessLevel?: AccessLevel;
+}
+
+/**
+ * Widget access level.
+ */
+export enum AccessLevel {
+  /**
+   * Default, no access to Grist.
+   */
+  none = "none",
+  /**
+   * Read only access to table the widget is based on.
+   */
+  read_table = "read table",
+  /**
+   * Full access to document on user's behalf.
+   */
+  full = "full",
+}