Enhancement: Support excluding cloud-backed files on macOS

Previously, restic treated cloud-backed files (such as files stored on iCloud)
like normal local files, forcing a full download of placeholders and other
"meant to be cloud only" content during backups.

The `backup` command now supports `--exclude-cloud-files` (previously only
available on Windows) 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/5370
