Split from this issue.

Requesting support for If-Match/If-None-Match headers for GCS via the Python client library.

My use case:
My API is a thin layer over GCS (for storing profile images), and I only want to actually read the image data and serve it to the client if the actual content is different from their cache. They will specify an If-None-Match header to my API (as they won't know anything about GCS or generation numbers, kind of the point), so I just need to essentially pass this along to GCS. I don't want to either do the extra round tripping of reading the current file to check the ETag and then use the generation number from that in a subsequent request, or not use the client library.