Add Google one-tap authentication example (#3244)

This pull request is inspired by #3243 and shows how we can use Google
one-tap authentication with NiceGUI.

---------

Co-authored-by: Falko Schindler <falko@zauberzeug.com>

.gitignore

@@ -10,3 +10,4 @@ venv
 .nicegui/
 *.sqlite*
 .DS_Store
+.env

examples/google_one_tap_auth/main.py

@@ -0,0 +1,46 @@
+#!/usr/bin/env python3
+import httpx
+from fastapi import Form, HTTPException
+from fastapi.responses import RedirectResponse
+
+from nicegui import app, ui
+
+# Get your Google Client ID from the Google Cloud Console.
+# For local development, you should add http://localhost:8080 to the authorized JavaScript origins.
+# In production, you should add the domain of your website to the authorized JavaScript origins.
+# See https://developers.google.com/identity/gsi/web/guides/get-google-api-clientid#get_your_google_api_client_id.
+GOOGLE_CLIENT_ID = '484798726913-t4es9ner8aglom3miptbnq1m23dsqagi.apps.googleusercontent.com'
+
+
+@ui.page('/')
+def main_page() -> None:
+    user_data = app.storage.user.get('user_data', None)
+    if not user_data:
+        ui.add_head_html('<script src="https://accounts.google.com/gsi/client" async defer></script>')
+        ui.html(f'''
+            <div id="g_id_onload"
+                data-client_id="{GOOGLE_CLIENT_ID}"
+                data-login_uri="http://localhost:8080/auth">
+            </div>
+        ''')
+        ui.label('Sign in with Google One Tap')
+    else:
+        ui.label(f'Welcome {user_data["name"]}!')
+        ui.button('Logout', on_click=logout)
+
+
+def logout() -> None:
+    del app.storage.user['user_data']
+    ui.navigate.to('/')
+
+
+@app.post('/auth')
+async def google_auth(credential: str = Form(...)) -> RedirectResponse:
+    async with httpx.AsyncClient() as http_client:
+        response = await http_client.get(f'https://oauth2.googleapis.com/tokeninfo?id_token={credential}')
+    if response.status_code != 200:
+        raise HTTPException(status_code=400, detail='Invalid token')
+    app.storage.user['user_data'] = response.json()
+    return RedirectResponse('/', status_code=303)
+
+ui.run(host='localhost', storage_secret='here you should pick a random secret string for your app')