Add more formats for `zb store object export`

[zombiezen]

Currently the output of zb store object export is the modified nix-store --export format that is a stream of NARs. This is useful for store-to-store transfers, but isn't as useful for users. I'd like a couple extra formats:

• zip/tarball of the relevant store object files, as if the top of the archive is the store directory. This would be useful for deploying artifacts to a machine that does not have zb installed.
• zip/tarball of the store objects in a binary cache layout. This would be useful for uploading store objects to a binary cache in workflows that are more advanced than S3/GCS realization uploading #165.