Support non-buffered streams for 'ResumableUpload'.

[krishna-kashyap]

Hi,

In get_next_chunk() method the stream is concluded to have ended on the read data being less than chunk_size rather than a empty chunk.

    if total_bytes is None:
        if num_bytes_read < chunk_size:
            # We now **KNOW** the total number of bytes.
    total_bytes = end_byte + 1

This could be a cause of concern especially in case of dynamically generated data (where nothing is written to file and data is being directly uploaded to GCS)

[tseaver]

Per the Python docs for io, "Interactive" streams as well as sockets / pipes can return "short" reads even when not at EOF. Supporting such streams for chunked / resumable uploads is a challenge: we need to be able to pass along a valid Content-Range header with each chunk.

[offsetcyan]

I've just (mistakenly, I suppose) raised this at googleapis/python-storage#1185. The last update in 2019 references interactive streams - this can at least be fixed with respect to non-interactive streams by calling isatty(). It also allows for a clearer error message to be raised, that interactive streams are not supported.