addUserHandler

@param identifier Handle handler unique Identifier, Used for subsequent Remove
```kotlin
NCEngine.addUserHandler("USER_HANDLER_ID", object : UserHandler {
    override fun onEventChange(events: List<SubscribeInfo>) {
        events.forEach { event ->
            println("user ${event.userId} Status change")
        }
    }

    override fun onFriendAdd(directionType: FriendDirection, userId: String, name: String, portraitUri: String, operationTime: Long) {
        println("Friend added: $userId")
    }

    override fun onFriendDelete(directionType: FriendDirection, userIds: List<String>, operationTime: Long) {
        println("Friend deleted: $userIds")
    }
})