Enhancement: Add `--exclude-cloud-files` on macOS (e.g. iCloud Drive)

Restic treated iCloud-backed files like normal local files, which forced a
full download of placeholders and other cloud-only content during backups.

The `backup` command now supports `--exclude-cloud-files` to skip those files on
supported macOS versions. From Sonoma (macOS 14.0) onward the option can prevent
unwanted downloads. Older macOS versions will still download the files during
a backup run.

https://github.com/restic/restic/issues/5352
https://github.com/restic/restic/pull/4990
