feat: support login for the development app

this fixes the local development issue with new backend system where the
user is not logged in thus resulting 401 errors from all api endpoints.

adds possibility to also add other login providers for testing plugins
independently.

Signed-off-by: Heikki Hellgren <heikki.hellgren@op.fi>

plugins/notifications/dev/index.tsx

@@ -18,12 +18,11 @@ import { createDevApp } from '@backstage/dev-utils';
 import { NotificationsPage, notificationsPlugin } from '../src/plugin';
 import { NotificationsSidebarItem } from '../src';
 
-// TODO: How to sign in here as guest user?
 createDevApp()
   .registerPlugin(notificationsPlugin)
   .addPage({
     element: <NotificationsPage />,
     path: '/notifications',
-    sidebarItem: <NotificationsSidebarItem />,
   })
+  .addSidebarItem(<NotificationsSidebarItem />)
   .render();