Support Multi-Account Push Notifications on a Single Device

Introduce support for additive push token registration in the SDK to enable simultaneous push notifications for multiple logged-in users on the same device. Currently, when the same FCM/APNs token is registered for another user via registerPushToken, it gets reassigned from the previously registered user, causing only the most recently registered account to receive notifications. This limitation impacts applications implementing in-app account switching or multi-account experiences where users remain signed into multiple accounts concurrently. The requested enhancement would allow a single device token to be associated with multiple CometChat users at the same time, ensuring independent push delivery for each account and enabling seamless multi-account notification support similar to modern messaging platforms.