Add support for ProgressBars to eta.core.storage

[brimoor]

It would be nice to have the option to render a progress bar when uploading/downloading files via the eta.core.storage module, to provide a better experience when transferring large files.

For HTTPSStorageClient, we use requests, so it should be easy enough to support progress bars via the approach outlined in:

eta/eta/core/web.py

Lines 136 to 146 in 03d3101

	r = self._get_streaming_response(url, params=params)
	etau.ensure_basedir(path)
	size_bytes = _get_content_length(r)
	size_bits = 8 * size_bytes if size_bytes is not None else None
	with etau.ProgressBar(size_bits, use_bits=True) as progress:
	with open(path, "wb") as f:
	for chunk in r.iter_content(chunk_size=self.chunk_size):
	f.write(chunk)
	progress.update(8 * len(chunk))

boto3 seems to support callbacks for upload/download functions, which should make this pretty easy for AWSStorageClient:
https://stackoverflow.com/q/41827963

Google Cloud doesn't seem to (yet?) support callbacks, so GoogleCloudStorageClient will take a bit more work:
googleapis/python-storage#27