cli: Add --cloudsync command

[RobLoach]

This adds a command to allow syncing cloud directly from the CLI.

> retroarch --cloudsync
[INFO] [CloudSync] Begin succeeded.
[INFO] [CloudSync] Server manifest fetch succeeded.
[INFO] [CloudSync] Created manifest with 239 files.
[WARN] [CloudSync] Opened local manifest.
[INFO] [CloudSync] Created manifest with 221 files.
[INFO] [CloudSync] Created in-memory manifest of current disk state with 223 files.
[INFO] [CloudSync] Finished processing manifests.
[INFO] [CloudSync] Wrote "/home/rob/.config/retroarch/downloads/manifest.local".
[INFO] [CloudSync] Finished after 2.521954 seconds, 0 files uploaded, 0 files downloaded

Fixes #18847