Added missing .env.local.example file for with example firebase-authentication-app (#14235)

Added missing .env.local.example file for with example firebase-authentication-app.
I've tested it personally and it is working fine.

examples/with-firebase-authentication/.env.local.example

+# Update these with your Firebase app's values.
+FIREBASE_CLIENT_EMAIL=my-example-app-email@example.com
+NEXT_PUBLIC_FIREBASE_PUBLIC_API_KEY=MyExampleAppAPIKey123
+NEXT_PUBLIC_FIREBASE_AUTH_DOMAIN=my-example-app.firebaseapp.com
+NEXT_PUBLIC_FIREBASE_DATABASE_URL=https://my-example-app.firebaseio.com
+NEXT_PUBLIC_FIREBASE_PROJECT_ID=my-example-app-id
+
+# Your Firebase private key.
+FIREBASE_PRIVATE_KEY=some-key-here