Support downloading a snapshot from LiteFS Cloud

[quells]

It would be nice if the litefs CLI could download snapshots from LiteFS Cloud. Nothing fancy - just the latest snapshot as if it were another readonly node coming online. This would allow Fly.io customers to backup their databases to another storage provider.

I've got a super rough POC here pieced together from bits from this repo and the LTX library: https://github.com/quells/litefs-dl

Apologies if I've simply missed something - I couldn't get litefs export to work. Although if I understand correctly, you're supposed to mount and then export? But mount doesn't work on macOS.