Add `sync_address_path` option to hide address in config

[yzx9]

I'm using a self-hosted setup, and my config file is world-readable. Therefore, I’d prefer not to expose the sync_address directly in the config.

It would be great to have a sync_address_path option that points to a file containing just a single line, e.g. https://api.atuin.sh. This would help keep the config clean and more secure.

If this sounds useful, I’d be happy to open a PR for it.

Thanks for your great job!

Alternative idea

• Store the sync address together with the key, so that the sync_address can be safely removed from the config. However, this approach would require designing backward compatibility for existing key-only files.