FiskalyHttpResponse should provide a way to access the response body as a stream instead of a byte array

[nicolasguenther]

This is crucial for handling downloads of DSFinV-K exports (https://developer.fiskaly.com/api/dsfinvk/v0/#operation/getExportDownload). We want to basically pass the response stream through our application without being forced to hold the potentially large export file in memory. Retrieving a fiskaly download url for the export file is not an option as we need to handle access permissions on our own.