[firebase_storage] Add support for getUploadSessionUri / putFile with existing Session Uri

[gusmd]

On Android, a developer can get the session URI of a pending UploadTask using TaskSnapshot.getUploadSessionUri(), and subsequently call putFile with the overload that accepts an existingUploadUri parameter in order to resume the upload session.

This doesn't seem to be supported in Flutter, which makes it impossible to resume an upload across app restarts. Is there a reason this isn't possible in Flutter?

[gusmd]

I've found this is a duplicate of #5564