New Features

• feat(fcm): Add live_activity_token to APNSConfig (#880)
• feat(fcm): Add send_each_async and send_each_for_multicast_async for FCM async and HTTP/2 support (#882)

Miscellaneous

• [chore] Release 6.9.0 (#885)
• refactor: Optimize success count calculation in BatchResponse (#837)
• chore: Use mock time for consistent token generation and verification tests (#881)
• chore: Correct x-goog-api-client header logic (#876)