Append "rpi-imager/version-num" to the default User-Agent when curl is
downloading the image so that image hosts can more easily distinguish
rpi-imager traffic from other traffic.
- Use larger buffer size when writing uncompressed files
(was libcurl's default of 16 kb, change it to 128 kb)
- Uncompress the next MB of data, while it is hashing/writing to
disk in seperate thread.