Add resumable downloads for llama-server model loading (#15963)

- Implement resumable downloads in common_download_file_single function
- Add detection of partial download files (.downloadInProgress)
- Check server support for HTTP Range requests via Accept-Ranges header
- Implement HTTP Range request with "bytes=<start>-" header
- Open files in append mode when resuming vs create mode for new downloads

Signed-off-by: Eric Curtin <[email protected]>

Author: ericcurtin