Add support for other destination types

[kytta]

Since you're building the repository URL from "blocks" (hostname, port, user, path), and hard-code SFTP in the process, it is impossible to use other destination types, like S3/Backblaze buckets, cloud providers, or rclone remotes.

Why don't we ask the user for RESTIC_REPOSITORY directly (expecting a URI)? We could then add a field for the needed environment variables, be it password or other setup parameters