gs-download-utils: Support If-Modified-Since caching
The download code already has support for ETags, but in case the server doesn’t support them, or the client doesn’t support the xattrs required to save an ETag alongside its file, it’s useful to support cache queries based on the modification time of a file.
Add support for Last-Modified
and If-Modified-Since
. This changes
the download APIs, so bumps the API revision.
Signed-off-by: Philip Withnall pwithnall@endlessos.org