[FR] Export Message types so they can be re-used

[BorntraegerMarc]

Is your feature request related to a problem? Please describe.
Take the following code:

import { TokenMessage } from 'firebase-admin/lib/messaging';

const allPushTokenSubscriptionMessage2DArray: Array<Array<fcm.messaging.Message>>; // Contains all the TokenMessage's I'd like to send a FCM message to. It's 2D because the max. nr. of tokens that is expected is 500. So if we want to send to 501 devices then the last device is in the next entry of the 2D array.

const batchResponses: Array<fcm.messaging.BatchResponse | null> = await Promise.all(
 allPushTokenSubscriptionMessage2DArray.map(async (pushTokenSubscriptionMessageArray: Array<fcm.messaging.Message>) =>
 this.firebaseMessaging.sendAll(pushTokenSubscriptionMessageArray)
 )
);

batchResponses.forEach((batch: fcm.messaging.BatchResponse, batchIndex: number) => {
 batch.responses.forEach((res: fcm.messaging.SendResponse, responseIndex: number) => {
 // Based on answers here https://github.com/firebase/firebase-admin-node/issues/600 responses should be ordered as requests
 const token: string = (allPushTokenSubscriptionMessage2DArray[batchIndex][responseIndex] as TokenMessage).token;
 console.log(`Failed to send to token ${token}`);
 });
});

The problem is now that TokenMessage is no longer exported in this lib. So TS transpilation will fail with the error Cannot find name 'TokenMessage'.

Describe the solution you'd like
Export all types of Messages.

Describe alternatives you've considered
None.

Additional context
None.

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[BorntraegerMarc]

This issue does not seem to follow the issue template. Make sure you provide all the required information.

Sorry, I don't see what part of the template I didn't follow. Maybe I'm missing something. But the description should be pretty explanatory.

[hiranya911]

The problem is now that TokenMessage is no longer exported in this lib.

TokenMessage and other message types were never really a part of our public API. Even in the early days they were left out of the admin.messaging namespace. But it's probably ok to expose them from the namespace.