feat: export-aria2 and URL list file generation

[Circu1tI0N3rd]

I've added a feature to generate a URL list file for files and folders (especially folders recursively). The resulting URL list is compatible with aria2c input file format and is intended for downloading using aria2c -i command.

Syntax: kaput files urllist [OPTIONS] URLLIST TARGET

[davidchalifoux]

Thanks for contributing this feature! I pushed a follow-up commit with a few tweaks:

• Fixed the inverted --no-replace behavior.
• Preserved the user-provided destination path when sanitizing remote folder names.
• Renamed urllist to export-aria2.
• Clarified and reordered the positional arguments as <PUTIO_TARGET> <OUTPUT_FILE>.
• Expanded the help text.

The updated command is now:
kaput files export-aria2 [OPTIONS] <PUTIO_TARGET> <OUTPUT_FILE>