Extend some data sources with get_item_by_name and retrieve_item functions

[favyen2]

    def get_item_by_name(self, name: str) -> Item:
        ...

    def retrieve_item(self, item: Item) -> Generator[tuple[str, BinaryIO]]:
        # yield filename, blob_file
        ...

These can be optional functions to enable users to retrieve entire files from the data source without the ingestion component.