1576 Update firebase_core dependency (cla: yes)

1577 [in_app_purchase] Add consumable demo (cla: yes)

1578 [firebase_auth] Update to latest CocoaPod (cla: yes)

1579 [firebase_messaging] more graceful handling of failed token read on startup (cla: yes)

1580 [cloud_firestore] Update CocoaPod versions dependency (cla: yes)

1581 Fix break in firebase_analytics CocoaPod (cla: yes)

1582 [firebase_core] set user agent (cla: yes, flutterfire)

1583 [image_picker] Add return statement into the image_picker example (cla: yes)

1584 Migrated linkWithCredential function to FirebaseUser object instead of FirebaseAuth (cla: yes)