Add an option to set UPLOAD_POOL_SIZE and DOWNLOAD_POOL_SIZE

## What feature would you like to see?
Describe the feature you would like to add, ideally proposing a specific API.

I would like an option to set `UPLOAD_POOL_SIZE` and `DOWNLOAD_POOL_SIZE`, for the `UPLOAD_QUEUE_EXECUTOR` and `DOWNLOAD_QUEUE_EXECUTOR` in `StorageTaskScheduler`.
Currently, as I see it, those executors are initialize [when Firebase is loaded](https://github.com/firebase/firebase-android-sdk/blob/dbf5d014ab08cf3d86b6e31d2650afad451fdb3c/firebase-storage/src/main/java/com/google/firebase/storage/StorageTaskScheduler.java#L43), so I'm not sure where and what would be the correct place to modify those values.

## How would you use it?
Tell us how you'd use this feature in your app.

One of my clients needs to upload a batch of lots of small files.
On iOS where Firebase relies on the OS for the parallel limits, it allows much more uploads at a time, and uploading all the files together is fast.

On Android, currently the limit of `UPLOAD_POOL_SIZE` is `2`.([see this line](https://github.com/firebase/firebase-android-sdk/blob/dbf5d014ab08cf3d86b6e31d2650afad451fdb3c/firebase-storage/src/main/java/com/google/firebase/storage/StorageTaskScheduler.java#L35))
Uploading the batch in Parallel on Android is faster than uploading the files one by one, but it's still 5 to 10 times slower than iOS, as the limit is too low.

Maybe there's a reason for such a low value, as it might was useful for some apps in the past, but I think that letting the developers to optimize this aspect of the app is better.

Looking for a solution, I didn't find a mentioned of this issue on Github, but I found some posts on stackoverflow, like [this one](https://stackoverflow.com/questions/60163889/android-firebase-storage-sdk-maximum-parallel-uploads) that suggested to edit the source and rebuild it by ourselves.
It is possible as a last resort, but it's not sustainable to maintain it, and make it even more complicated when using the SDK in other frameworks and engines like Unity.