[FR]Use multiplexing over HTTP2 in sendEachForMulticast method for batch push.

Is your feature request related to a problem? Please describe.
A clear and concise description of what the problem is.
When I upgrade fcm to Http V1, the speed will become very slow when using sendEachForMulticast in firebase-admin:9.2.0.
Isn't HTTP/2 used in the SDK?

Describe the solution you'd like
Convert bulk send in SDK to HTTP/2.

Describe alternatives you've considered
Keep the original sendMulticast method.

Additional context
According to the database conditions, users (hundreds of thousands) who meet the sending conditions are selected for message push.
If the current sendEachForMulticast is used, it will be a nightmare.
The following code took 15 seconds to execute.

        List<String> registrationTokens = new ArrayList<>();
        for (int i = 1; i <= 500; i++) {
            registrationTokens.add(TOKEN + i);
        }

        MulticastMessage message = MulticastMessage.builder()
            .setNotification(Notification.builder().setTitle("test title").setBody("test body").build())
            .addAllTokens(registrationTokens).build();
        BatchResponse response = FirebaseMessaging.getInstance().sendEachForMulticast(message);
        System.out.println(response.getSuccessCount() + " messages were sent successfully");